Luis,

The Mitsui SxS were the same SKB guns imported by Ithaca, models 100, 200 etc. After Ithaca broke the arrangement Mitsui marked SKB's were sold here in the US with the same model numbers in the 70's to early 80's.

The SKB boxlocks are a loose copy of Webley and Scott 700 series guns. Changes made to simplify manufacture. Not sure if parts would interchange. Possibly with some fitting.

Chief