The 1954 Truck Data Book dated June 22, 1953 (an original) does not list the RPO 315 Overdrive Transmission option either. Oddly, that same book has many empty pages for the Sedan Delivery but all the Sedan Delivery pages have the model 1508 in the header and June 22,1953 in the footer but in the middle of the blank pages is "Revised page will be mailed when complete infortmation becomes available."
There is a PRICE INFORMATION insert in the back of this book and it is dated January 4, 1954. It does not show the RP0 315.
The late 1954 Truck Data Book (a reprint) that I referenced in the post above has entries from January 4, 2004 that refer to the 1954 enclosed driveshaft equipment and have no mention of RPO 315. However, the pages in the same Truck Data Book that are dated October 29, 1954 seem to refer to open driveshaft equipment found on 1955-1st series vehicles (including pictures of the 1955 new body-style Sedan Delivery).
So, it looks like the 1955 Automobile body style became available in late 1954. There was an open driveshaft on this model; and, 8-cylinder 1500 models autos are listed but there is no information about an 8-cylinder engine; and, the RPO 315 Overdrive Transmission is listed for the "late 1954" 1508 Sedan Delivery.
Bottom line: it looks like there were no GM RPO enclosed-driveshaft overdrive transmissions in cars or trucks.
