I bought a lockless Hatfield from a member here and a (second actually -- I have a spare for a Hatfield as well as a Pedersoli Frontier flint) L&R RPL lock. Fitted by a gunsmith -- most of the issues were that mortising had to be inlet a bit across-the-board even though the orignal was a little long for the new lock, the cock would have been impinging on the stock and wood had to be shaved and refinished above the lock, and finally the sear bar or arm was too short by a hair so the lock could not trip...
L&R replaced the sear with a longer armed one and re-tuned the lock. However, now the sear arm has to be bent a tad forward to engage the trigger blade which it only does begrudgingly now wrapped with alot of tape. In other words...
...they are NOT quite "drop in." Nevertheless, this will be as good if not a little better lock than the orignal and part of a "shooter" that can get dragged around!