Model 64 Question

[H P Bushrod]

Ran across a Model 64-1 .38 with the 4 inch bull barrel today that I put on lawaway at the local gun store. It's in excellent condition. I broke out my trusty copy of the SCSW to check the serial number. The number starts out D735XXX, but the book says there should be a 1 in front of the D. I was trying to figure out approximately when it was built and wound up with a headache. Any feedback guys? Thanks!

 

[bamabiker]

The engineering changes show the M64 started in 1970 and the M64-1 in 1972 before the 1D serial numbers that came in 1976. I think your's would be 1975. I know it don't show the M64s above the D serial prefix list but it would have to be, I guess, I think, I imagine.
If I'm wrong someone will correct me.