I read through you post on the drain plug and wanted to point out a few observations about the M46 Type II aluminum case transmission and the M47 5-speed, as they are the same for the first 4 gears and case.

The plug is not brass, it a soft zinc steel, probably softer than brass.
If the plug had never been removed before, which is common on these tranny's with high miles, it would still have the threadlock on the plug which can be softened with heat, by applying a torch to the case.
Volvo probably put in a very good quality of oil at the factory, just like the M47, but may not have given it a 'lifetime of the car oil change interval' like the M47 because of the need to have the OD unit flushed more often as you would do with a auto trans.