Actually most here are correct in a way:

If the Jeffery double was built before 1908 ( actually between 1890? and 1908) likely Thomas Turner Jr. as finnisher but definitely not after 1912 as the company then ceased operations. ( I have two reputable sources stating the same) Thomas Snr was mainly a barrel maker and he had two sons James and Thomas Jr. both gunmakers.

As to the bolt guns:

I have the copies of the original Jeffery ledger entries as they pertain to the 21 Jeffery 500's from # 1 delivered in 1927 to the final rifle and the ledgers shows all to be finnished by H Leonard.