Lloyd,
This, is an actual side lock:

[Linked Image from i.ibb.co]

There was never anything like it produced here in the new world.

Never.

Anything else built here was a simplification of the concept, that retained none of the advantages of the pictured lock (excellent trigger pull weights, intercepting sears, maximum amount of wood left in the head for strength) and brought a few disadvantages unique to North American production guns that claimed to be sidelocks, the criminally weak stock head of the ‘Smith being an example, and the weak frame of the Tobin being another. The only part on the side lock of a Tobin, is the sear and sear spring, everything else is carried in the frame.

What makes it a side lock?

They are, to a one, simple, cheap, designs, built to be afforded by new worlders who frequented hardware stores, not Abercrombie and Fitch. To me, any run of the mill English box lock is superior for regular use to any so called side lock gun built in the US during the golden age of doubles claiming to be a side lock.

Good luck with your project gun. You might need it.

Best,
Ted