The first one (see right) has two twisted cords as its
power source and the striker board is inserted between the twisted
cords. Mascall (1590) called it “Dragin trappe for Mice or Rattes”. Actually, the stiker board looks like a bat and the handle of the bat is inserted between the twisted cords.
If material culture is evolving, one question is how to get from a trap with twisted cords as power source to one with a wire spring. Going one step at a time, you would have to replace the twisted cords by twisted wires, in order to be able to insert the striker board between the wires. In my opinion, twisted wires would be a poor power source. Apart from that, I do not see how to get from two twisted wires with a striker board inserted between to one wire with curved or coiled ends serving as power source and a middle loop serving as striker.
The only one-step-at-a-time solution would be to
form the wire spring in such a way that it has a middle loop to which
the striker board can be fixed.
Searching for traps with this design
I found two patents.
The first patent is from William Wrigh (1860), Philadelphia, PA. The figure at the left is taken from that patent. The side view (Fig. 3) shows the striker board e with spikes being pressed down by the wire a. The top view (Fig. 4) shows that the wire a is actually the middle loop of a spring being coiled at both ends. The trapy type is L-shaped, which once was a large group of traps with considerable variability in design. 
The second patent is from Carl J. Erikson (1887), Rockford, IL. The figure at the right is taken from this patent. It looks complicated, but the relevant part is the striker board c, which is fixed to the middle loop of the spring wire B.
As an aside, part H is a bait box, an odd feature of this particular patent. The inventor thought that the mouse would be attracted by the bait in this box walking around it to look through the windows in the bait box. As the windows were at the side of the striker and the tradle, the mouse would release the trap when looking through the windows. The round base seems to be a heritage from the older dragon-traps (why
else should the base be round when actually a rectangular base would be
far more practical for accommodating the bait box?
Admittedly, these trap patents are from
a later period than the dragon-traps, but I suggest that they could represent a missing link in spring/striker design.
A striker board is fixed to the middle
loop of the wire spring. But now the striker board is utterly
redundant. The middle loop of the spring only needs to be formed in
such a way that it can carry the striker function on its own. Et
voilà, a Dragon-trap with spring/striker unit ‘evolved’.
There’s more to Dragon-traps. For example, the set-release mechanism changed from suspended to hinged striker.Therefore, a second missing link would have to look like the one above except for having a treadle and suspended striker instead of bait hook and hinged striker.
P.S.: The younger history of mouse traps shows that they
(and by implication other irreducibly complex systems) can have
precursors with one part less and therefore can evolve (see Behe's mousetrap nemesis).
Appendix: Mascall's originals
The Dragin trappe for Mice or Rattes is described as follows:
“This engine or trappe is made of wood like the stocke of a Bell, with two holes at the endes, and therein is put thorow [through] haire or corde double, and the lidde is put between, and so wreath the haire one way towarde the lidde uppon the under hoope, with sharp wyars [spikes] set round on the upper lidde, and a long badge under, falling within the neather hoope with a staffe set fast above on the stocke, to tie the string and clicket, which must holde up the lidde: the upper trappe lyes with the mouth towarde you, and the lower with the side towardes you.” (Mascall 1590, p. 71)Actually, both are top views rather than a front and a top view.
Mascall's drawing of the Dragin trappe with a great wyar (right) is better. For those who delight in old English, however, here is the explanation:“This engine or trappe, is to take Mice and Rats, it is made of two bordes: the neather borde is made round at the one end, and broad at the other end, like a swallow tyle. Then is there an other bord set on edge in the middell thereof, which borde hath a great wyar bowed and nayled thereon, as ye may see: and the rounde bowght thereof must lie close on the round ende of the neather borde. Then is there two short square bordes nayled and spreading toward the great wyar. Also yee see at the toppe of the upsight borde, a long mortis, wherein the wyar that must holde the clicket and baite, must goe thorowe and there staye on another wyar, and the like is holden up with the upper ende thereof: which clicket, doe stay on the toppe of the middle or upright borde, and so tyde thereat with a threede which holdes up the great wyar, when that ye will set or tyle him: also the neather borde is set rounde with sharpe wyars [sprikes] to holde mouce or ratte, and they must stande all within the bowght of the great wyar, and it is done.” (Mascall 1590, p. 74-75)
References
- Dagg J.L. (2011) Exploring mouse trap history. Evolution: Education and Outreach 4: 397-414.
- Drummond D.C. (1992) Unmasking Mascall’s mouse traps. Proceedings of the Fifteenth Vertebrate Pest Conference. DigitalCommons@University of Nebraska, Lincoln. Accessed on: 21 Sept 2008.
- Drummond D. (2005) Mouse traps: a quick scamper through their long history. Galloway: North American Trap Collectors Association. [David is the most prolific scholar on mouse traps, this link brings you to his website]
- Erikson C.J. (1887) Trap. Patent no. 365,058. United States Patent Office.
- Mascall L. (1590) A booke of engines and traps to take polecats, buzardes,rattes, mice and all other kindes of vermine and beasts whatsoever. In: A booke of fishing with hooke & line. Another of sundrie engines and trappes. London, pp. 51–93. [These are two books in one volume. The trap book starts at page 51]
- Wright Wm. (1860) Animal-trap. Patent no. 28,820. United States Patent Office.
