Cosmo, the reverse lockout is controlled by the governor on the very back of the overdrive unit. The governor does allow the unit to shift into overdrive at speeds under 28 MPH. The front box is totally independent from the overdrive unit. I think the extra hole is just there unless possibly it had something to do with a backup light. I think the link I provided earlier is password protected. This one might work.
http://www.webhubcentral.com/packard/downloadable/BorgWarner_Overdrive.pdf It's 20 pages so it takes a little to download.
