Tom, I agree with everything you said except I don't believe frame #'s came into being until 1951.
We are definitely going to need some pictures!!
The dual cap tank didn't appear until 1953, and the frame # is 900 earlier than my 53, but the engine was produced 4000 later. I wonder if it is a 53 manual shift bike that had an engine failure early in life, and Simplex replaced the motor under warranty?
The 3" tall riser with the key is correct for 52 and 53 models. I don't know when they stopped using that, may have gone on for a couple more years.
Rick