COTW shows a case length for the .38 Extra Long Rimfire of 1.490; for center fire, .357 Maximum cases could be trimmed or .357 Magnum cases used as is. (The cases - DO NOT try to fire normal loads for either cartridge in that gun.)
I don't have a Lyman catalog, but they used to show some heel type bullets that would have a small back end (the heel) to fit ID of the case, while having a front end the same OD as the case. That is the setup you need.
.38 Colt would be too small, and .38 S&W too big. You could use .38 Special but as was noted, that old gun should be used with black powder, so it is a handloading proposition.
Jim
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