Hope you find your water pump. When I found the correct 69 327 cid for my ride, it still had the original 3953692 water pump, K 8 8 date code for Nov 22 engine build.
In my searching around over the years, I do have a spare GM water pump. It is #3927170, date coded E 17 71. I realize that you are going for an accurate date coded rebuild and that this pump probably isn't what you are looking for - however, it is available.
Additionally, according to page 224 of Chevrolet by the Numbers: 1965-1969, it states the following about #3927170: This water pump was used interchangeably with #3953692. #3953692 saw extensive usage, #3927170 saw very little usage.