The following is copied from my blog. It is the post called "Vernier plates and counter washers" from Aug 25, 2015. Also the post called "Washer day" from Sept 5, 2015 has more on this subject. Thank you, Nick Watts.
It is clear to me that the four holes seen in the end caps of the field frame on the Elenovo artifact are too close to the spring hole to be utilized for the locking pins for the washer. Here's a Drawing of the Elenovo end cap by Kayumov & Minchev (2010):
The locking pin holes in the washers must reside on the perimeter of the rim, just as they are on the Hatra machine (photo from Baatz 1978):
The basic design of any ballista washer requires the locking pin holes to be in the rim near it’s outer edge. The four holes in the Elenovo end cap would be obscured by the vertical walls of the washer because these four holes are so close to the spring hole. Clearly any holes utilized for locking pins would need to be much nearer the edges of the end cap where the rim of the washer actually rides. Unless, of course, one postulates an impossibly small spring diameter and tiny, little washers. That would be pretty self-defeating wouldn’t it?
And to clinch the argument, it appears as if the lower locking hole in the end cap seen below is not concentric with the spring hole. It is out of position axially and would not line up with any hole pattern put into a rotating washer. However, for locking down a vernier plate that does not rotate, it would work just fine. (A vernier plate is simply a flat plate shaped like the end cap. It has four pins projecting from it so it can index itself to the end cap. It is this additional plate that contains the hole pattern used to create the 7 1/2 degrees of incremental adjustment. It is also a handy way to mix and match frames and washers because, as a third wheel to the party as it were, it’s holes can match whatever asymmetrical silliness is going on in the end cap with the pattern of locking holes in the washer. In short, the vernier plate can contain all the intelligence needed for a proper alignment between otherwise mismatching frames and washers. A very handy feature if you are running an ancient catapult factory.)
The upshot of this reasoning is that the vernier plates I utilized on Firefly make perfect sense for the Elenovo machine we will be building. This may seem like a trivial point not worthy of such attention, however it is these small clues that light the way on how the original machine must have looked. Perhaps it is my terminology that causes reticence in certain quarters. “Vernier plates” are pretty much the same idea as the well accepted “counter washers” seen on wood frame machines.
My posting from Dec 25, 2008 makes the basic case as I saw it on an early version of Firefly. Now, the Elenovo artifacts make it plain.
And finally, a photo of a disassembled field frame with my trusty vernier plates laying there on the right.
Note the pattern of locking holes that act like a vernier scale to allow the 7 1/2 degrees of incremental adjustment for twisting up the springs. The spring hole in the vernier plate is of larger diameter that the one in the end cap of the field frame. This provides a recess for the projecting flange on the underside of the washer to ride in. This in turn allows the torsion spring to be as large as the spring hole in the artifact. Not a trick, just no wasted space. A maximization of what is already there within parameters the Romans readily had available. All perfectly legal in this game.