Greetings. Picked up a Model 1899 chambered in .22 HP. Not really sure which version it is as identification so far has been very confusing. S/N is 6 digit starting with 261.

Not looking for value, just trying to understand what it is. It's been fitted with a rubber butt plate. Has also been drilled and tapped with scope rings. I think it is a takedown model as there is a latch on the fore end for easy removal and the barrel twists off after loosening two set screws. There isn't a marking on the frame once the the forend is removed. Hoping to understand when it was made and what "model" of 99 it is. Forward of the rainbow colored lever (case hardened?) is the letter S. Thanks.