Anyone know when Oberndorf 98 Mausers went from having only the "B" and "U" stamps to the B, U, G, and N stamps?

Finally a question I can answer; the answer is: 1924; and they added the N in 1934. BUT, they had lots of parts in stock with the old proofs so you will find rifles with no N mark which were assembled at later dates.
I am sure you know that G means a rifled barrel, and N means Nitro and were added due to German proof law changes. They changed again in 1940 and used the eagle N and the Oberndorf proof house used an elk/stag antler mark.

Thanks, DPCD. The receiver has the older WAFFENFABRIK MAUSER OBERNDORF A/N roll stamp, so I knew it was at least early 1920's. Do those have any date codes underneath that will tell the year of manufacture?

Yes, on the barrel.