Mine seems to be #38665. If they are numbered sequentially, depending on how many they sold per year, it must have been new sometime in the 70s. I guess the previous owner is getting his mowers confused, nevertheless, I'm still enjoying rebuilding both of them.
The next thing to do with this one is to remove the cutting assembly and get it sharpened and reset.
Thanks again for all of your help - it's really interesting. For someone with so little lawn, I now have three mowers, including my old Rover.
Chris