Paul:
Is yours not a pre-12 with the C prefix?
On another forum recently, the bug screw monicker was thoroughly discussed and I believe it was decided that it was the large upper screw that was actually called the bug screw as it is shorter than the others and was a bugger to get threaded. The smaller screw was probably to keep the bug screw from unthreading. As I remember the discussion.
Ed
Just the opposite; the large short screw is the upper sideplate screw (most often seen w/o the smaller screw) and notched for locking in place by the smaller or "bug" screw when it is present (as shown below).
I believe "bug screw" is a fairly common term among machinists to describe the bug-like appearance of a second, smaller screw used just for locking its larger partner in a specific postion. In the case of the Air Weight, I would think use of a bug screw would have probably been useful to discourage cranking down too hard on that top screw and stripping out the threads in the aluminum frame... JMHO.
"Bug Screw" is a rather ambiguous term. You won't even find a definition in the
American Machinists' Handbook.
So, we are left with traditional usage and colloquial definitions of the term.
All my life, I have heard it used for screws that
lock other screws
or parts. Decades ago I first heard it used to describe the screws that lock the trigger guard screws of Mauser rifles.
Roy Jinks, now the factory Historian and formerly Production Manager of S&W calls the large upper sideplate screw the "bug screw", so we can assume that terminology is acceptable at the factory!
Parts catalogs of the pre-war era call them:
Plate Screw (large head)
Plate Screw (small head)
One can see how the factory personnel would prefer 'bug screw' to 'large upper sideplate screw' for conversing day in and day out.
This is a reasonable application of the term.
Note that it functions in a manner different from the other sideplate screws, and most screws in general. Think about it- most screws go
through one part and into another which is threaded. This upper sideplate screw does not. It actually clamps or
locks the upper sideplate corner in position by threading directly into the frame and clamping the sideplate with its large overhanging head that descends into the countersink.
The gun pictured above has:
a bug screw that has a bug screw.