Hi, Hawg and folks,
When it comes to shooting those Model 1877's, the load and/or powder is not really the problem. Obviously, the guns are old and light loads are recommended, but the big problem is simple parts wear and/or breakage. The design is very complex (unlike the SAA) with odd shaped and fragile parts that are impossible to make at any reasonable price. A trigger for an SAA is essentially a piece of steel with a hole and a slight bend. The trigger for a Model 1877 is a very complex piece of machined steel; I can't imagine anyone making them today the way they were made then for less than $500 each. Even trying to make them by investment casting would, I think, be extremely difficult and costly, with a lot of hand fitting still needed.
The number under the trigger guard is probably an assembly number used to make sure the loading gate gets back with the right frame after finishing; if so it will match only the gate number, not any other number.
Jim