That 16 gauge should be on the FW frame- not the R or std frame- the two extra mounting holes with machine screw heads at the 'tongue" part of the lockplates, and the smaller bottom barrel lug are the "look-fors" here. I would check the chamber depth first- you can use 2.5" RST shells, better move IMO that opening shorter chambers- any cracks around the lock plates or tangs- if so, be advised that restock an LC Smith sidelock with a newer buttstock will rip through your 401K like Bernie Madoff pilfering a Hedge Fund.
