[rotorwrench]

I'm fairly well versed in the 49 thru early 51 Merc cogboxes and they used the same intermediate gear and low/reverse slider as the earlier Ford 3-speeds 51A and 21C prefixes respectively. Just the main output shaft & case was different and there were the two types. Trans with overdrive and trans without.

It's the later 51 and on stuff that is all different with respect to Ford & Mercury to early 3-speeds.