The thread on ASM of that time frame is SAE 12-28. If you like the "nipple solution", use the nipple size labelled as "5.5mm x 0.9mm", which is, of course, SAE size #12, 28 TPI.
44 Dave, good idea on using nipples! However, from your picture, looks like you have an early, 1969 mfg, Uberti...