I have never been able to find a definitive answer as to the difference in M20 trans. options for RPO L48,L34,L35 & lM1(3950356) vs. Z28&L78(3946797). I reference the AIM M20 A1. Hoping someone will educate me as to How to ID one from other when tag on side cover is missing. Somewhere I thought I read it had to do with speedo gear, but that may explain the Z rear axle, and not L78.... see my post regarding "SS 396 rear axle ratios" below. I hate to post if this has been covered already, but I pour over these posts daily and do a lot of searching and cannot find the correct information. I promise, I will buy Jerry's book someday, but until then? I guess, I too would like to be regarded as an "expert" someday on at least 69 Big Block cars. Thanks again CRG, Bentley B.