Posting for Google-ability. My 1985 Centurion IronMan rear dropout positioning screws are threaded M3-0.5. M3-0.5 x 20mm seems to be a fine length. Less robust than a proper hardware but works in a pinch for pennies.
Ironically, I thought I needed them but with the 11-28 freewheel I swapped in, the axel needs to be virtually slammed back else the chain can get caught. Shifts fine in that position and Sheldon suggests “don’t use the dang things.” Glad I didn’t drop $25 on a proper set from eBay.