Chances are slim that someone may have a Rigby style forend grip catch lying around but strange things sometimes happen.
Have an Alex Henry .450 takedown rifle in the shop missing the entire forend.
Pictured below is a similar forend catch from a Watson Bros double rifle which is very close to working with the barrel lug on the Alex Henry rifle. I am sure it could be made to work with a bit of precision welding/shaping to the barrel lug.
Please use P/M if you may have something.
Thanks,
Jim