Closest I have to your S/N 98488 is 99052 4/1921 (I also show a couple of M&Ps having slightly lower SNs which also shipped in 4/1921). .32-20 M&P dating from the SN comparison alone is somewhat unreliable as they were slow sellers and could sit in factory inventory for awhile prior to shipment, but yours should have shipped within a year or so of 1921. It's certain yours wasn't made later than 1921. Yours should not have the "MADE IN U.S.A." stamp on the right side of the frame, and probably has the S&W logo stamped on the left side.
High velocity Rifle ammo in .32-20 has not been commercially loaded for well over 50 years, so you are very unlikely to find any. Anything which has a lead bullet will be safe to use. Remington boxes are printed "Rifle" but they really aren't, and are OK for your M&P.