This is my 18247 which was made about December 1941.
Your 16643 was made in 1940. It is incredible what beautiful wood the put on these "Standard" grades.
Model 21 numbers don't necessarily follow each other linearly. Putting an accurate year date to a serial number is tenative at best.
According to the paperwork from Cody Firearms Museum, my Model 21, serial # 168** went into final inspection on Feb. 2, 1942 and completed final inspection one week later.
The only way to realy know for sure when a Model 21 was built is to get a letter from Cody. It's like a birth certificate. Well worth it with the cost of 21s being what they are these days.
Just some thoughts.
Tony