I believe all 19th century side lock parts were forged wrought iron.
Malleable cast iron was available, and used for things like Allen & Thurber pepperbox frames, London Colt "steel" grip frames & trigger guards. But malleable iron is not worth a @!*% for a percussion cock, anything that gets repeatedly pounded on.
In the late 1950's Turner Kirkland made a number of cocks &c from malleable cast iron. This stuff has some ductility, as compared to the grey iron from which old pots & pans were made.
Investment cast steel lock parts have been available as long as I've been messing with this stuff, from the early 1960's anyway. Investment cast steel is, I believe, a 20th century development (yes, the process of investment casting itself is thousands of years old, but not where steel is involved. Jet engine production helped develop the modern process for investment casting steel and nickel base alloys).
I have no information of my own with respect to who made Leman's barrels.
However, on page 159 of The Pennsylvania Kentucky Rifle, Henry J. Kauffman says "...Henry Leman of Lancaster is reputed to have had the first boring mill that was operated by a steam engine." P283 illustrates a trade gun with barrel imported from Birmingham, England. Page 284, discussing Leman: "In 1834 he returned to Lancaster and started his business of gun making, which appears to have flourished until 1887. His first factory was on Mifflin Street, west of Duke. His forge and boring mill were in Upper Leacock Township, along the Conestoga Creek...In this mill he welded, drilled and ground the rifle barrels"
Guess this means ol' Henry made his own barrels.
By the way, if you are interested in Kentucky rifles you must get Henry Kauffman's book. It is a detailed work done by an intelligent man and one may, in my opinion, trust that he was careful about his sources.
One might rely on Kauffman's veracity.
