The serial number does indicate a Model 1900, and it would be 7.65mm. A sure clue is that the toggle knobs are not flat on the side but are "dished" and the right toggle knob will have a spring loaded catch in it. It probably was made around 1901.

Thank you Ron.

The toggle knobs are "dished" and the right one does have the catch in it.

I'll check the caliber tonight, but I am almost positive it will be 7.65.

Thank you very much for the timely information! :)

Best regards,
Cp