"Took off the grips and absolutely no markings of any kind on either grip."

Not unusual for the 1920s grips. SNs were penciled, and often very difficult to make out - if you can see anything at all. They have value and shouldn't be thrown or given away.

Regarding SN 658458, I'd say 1936 is the most probable shipping year, but a year earlier or later is not out of the question. You did pretty good on the price.