I seem to remember some transmission fluid filters being attached with that type of Allen head machine screw, at least on some of the early model Turbo 350's. From the general proportions of the screw compared to the dipstick piece, it looks like either a 10-32 thread, or maybe a 1/4"-28. Look at the valve body to case attaching bolts- - - -they're usually 1/4" hex head bolts, so I doubt if Allen screws would be used there.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!