Marbles suggests measuring the distance between the two drilled and tapped screw holes, my Italian Armi Chiappas Sharps 1874 was about 2 and one quarter inches.
From Marbles website they make bases in 2.283 inches (58mm) fixed holes (009836/009838), and also 2.000 thru 2.265 inches range with one fixed and one slotted hole (009816/009843). Some may prefer the fit and appearance of fixed holes over the one-size-fits-most convenience of slotted holes.
If one has a combination metric and regular screw thread gage in their tap and die set, or even some small screws of different threads (8-32, 10-24, 10-32, chinese, etc.) just laying around, then perhaps the hole plug screws can be measured too. Number 10-32 seems to be somewhat common.
I ordered the fixed 2.283" from Midway (466302), and apparently received the slotted hole version
, but it fit alright (only 0.018" difference) front-to-back, but seemed to lean back a few degrees into shooters eye, not exactly perpendicular to barrel or line of sight
.
Other Midway posters stated that what they received fit their Petersoli Sharps, but associated screw sets may have not fit.