The 84-88 4+3's gear box section, essentially a beefed-up Warner Super T10 was an ok transmission, but the electro-hydraulic overdrive was a complete POS. Doug Nash sold GM a bill of goods on that deal. As a result, GM's warranty costs for 84-88 manuals was obscene. The overdrive was under continuous developmental change and it wasn't until the final year, 1988, that the OD approached acceptable reliability/durability, and by then Nash was bankrupt.
There were two reasons the ZF S6-40 six-speed was developed 1) because the 4+3 was a piece of crap and 2) the coming ZR-1 needed a transmission with a much higher input torque rating.
Even when the Nash overdrives were properly maintained, they still were a reliability/durability nightmare. Granted some had reasonably good luck with them, but Nash's quality control was so freakin' inconsistent that for every good 4+3 out there there was one bad one. But, then, there was some Karma. Trying to support that crappy overdrive combined with mis-management put Doug Nash out of business. Richmond gear bought some of Nash's aftermarket transmission designs, finished the development on them and sells the products today.
As for the RPOs, the merchandising RPO for 84-88 manuals was MM4. Apparently, there were two different OD ratios, so there would have been engineering RPOs, MK2 and MH5 for each transmission.
85-88 Service manuals say the OD ratios are either 0.68 or 0.59. GM parts info from back then is conflicted. In the ID section, it says only the MK2 was available for MY84-86 with both MK2 and MH5 available from MY87-88. In the parts listings it says MK2 was stand-alone only in MY84 with either one available in MY85-88.
So far, I can't find info on what cars had which OD, but my wild-assed guess is that one OD is the early design and the other is at the later, improved design. For some reason the ratios differ by 0.09:1. Interestingly the speedometer gear numbers for both are the same.
