I don't know if this applies here, but I did hear about this type of thing in the past.
Most manufacturers have a Quality Assurance program in one form or another. Maybe every fiftieth (or whatever number) gearbox was pulled off the line and hooked to a mule set-up. Here it was run through the paces, and when all went well it got stamped and was noted.
If gearbox number 250 passed, but 300 did not for whatever reason- they knew they only had to check back 49 boxes (more then likely number 275 would have been checked right off to split the difference).
Maybe those XX marks were the Q.A. passed symbol. I know some early AMC (auto) rear axles had a capitol "I" on each side of the serial number- and this caused huge confusion for restorers!
Lee