Numbers are the raised casting marks, they have no relation to dates, but, depending on the type of extension housing as to what models they were fitted to.
I tend to ignore them, as over the last 30 plus years many have been dismantled and renewed, by the many gearbox shops, and I see plenty that has me wondering why it was done? Ie a 720 truck ratio set put into a Bluebird box, or a early old style synchro set put into a DR30 Skyli box. Have seen a few so called early 260 boxes ( with the fill plug on rhs) that have plain ratios ie 3.32 first instead of the desirable 2.9 first.
Later extension housings with the external horizontal ribbing has a better reverse "lockout" mechanism on the main selector rod, but the early extension housings have much more support metal right at the back to prevent the tailshaft from flogging and ripping the very back off.
