A little serial number mystery

[SteelHurts]

So I am looking on my frame for the serial number which I found stamped on the drivers  side just ahead of the rear motor mount area. It reads M15A1-1374.  But on the data plate on the dash it has some other numbers that I want to know from you guys if you can clear up or educate me on. On the top left of the plate it is stamped 3-53484 crowded in near the upper left rivet hole. Then it says (printed not stamped) CARR,MOTOR,MULT.GUN M-16-A1  then stamped directly to the right of that is RDB.

My question is- what to those two stamps indicate?  I will try to post a picture of the data plate.  Did it start out as an M15A1 then was rebuilt into a M16A1?  When? Do those numbers signify a date?  The Mfgrs number stamped on the data plate matches the frame number as does the Ord number both are stamped in the data plate as 1374.
I looked at the g503 list, but quite honestly didn't really follow it much, Hey Im a newbie!
If anyone can edumacate me a bit I would appreciate it.

[SteelHurts]

[WayOffTrack]

Did it start out as an M15A1 then was rebuilt into a M16A1?  When? Do those numbers signify a date?

Yep you are correct. Started out as an M15A1 then converted to M16A1 in 3-53. There has been some speculation that RDB is the initals of the inspector. The 484 means it was the 484th M16A1 conversion ( I think). I have a plate that came off an M16A1 my dad owned, it is #298 but the date is 5-53 and the ord# is unstamped. It was converted from a Diamond-T M3 to M16A1. I don't think very many of the 16A1's made it to Korea before the war ended. Perhaps it was held back on the coversion line for some reason/problem since the number is lower than yours but the build date 2 months later.