w houle: A Model 10-9 will have a serial number starting with a 3-letter prefix, followed by 4 numeric digits. The serial number is always stamped on the butt in modern S&W revolvers, and often appears inside the yoke cutout as well. Other numbers stamped in various places on the gun (usually in places you don't typically see them, such as the frame inside the yoke cutout or on the back of the cylinder) are most often "assembly numbers" that are used to keep track of fitted parts during manufacture and assembly. However, the number you list would be an unusual assembly number, in my experience - where is it located?
SRTTECH: You have a .38 Special Hand Ejector Military & Police (which became the very popular Model 10 when S&W went to assigning model numbers) from the 1915 to 1942 time period. That serial number would place it toward the end of that range. The other number you listed (21506) is almost certainly the assembly number (see above).
ETA: SRTTECH - my comments assume (without facts in evidence) that it's a 6-shot. If that's not the case, then please ignore.