I think Winfer's data is about as accurate as can be found.
Every gun I've owned of the type he published on matched up pretty well.

Jeffery didn't make any Farqs until 1895 so it can't be older than that.
Also, the 303 didn't go cordite until about that year so if it has cordite proofs, 1895 is a good guess.

Some makers would use hold over serial numbers and WR actually renumbered actions as they were used to fit into the new series. I had a WR double which the ledger tells was renumbered this way.

That said, I see no reason for Jeffery to back number a new action of new design.

Let's see some photos!