2500?2 is in the series of M4 mortar carriers all made by White so definitely was not an M15 it would seem. If it was an M4 modified into an M16A1 that would be VERY interesting. When you can post some pics of the rear part of the frame. If it is a long frame with that serial number it might be one of the few White build M3's. Might be a rare beast, congrats!