Fuzzball:
Assuming no letter prefix to the serial number, then it is a .38 Military & Police Model of 1905 4th Change manufactured in 1941. It lacks a positive internal hammer block safety and should be treated as a 5 shooter, with the chamber under the hammer left empty. If there is a letter prefix (V, SV, S, C, D or K) then it is a later variant. If so post the prefix and I'll give you a date.
missyoak:
Confirmed for a transitional model - has the long action hammer, post war front sight, pre-war made in USA stamping instead of 4 line address. The grips are probably fake mother of pearl. If they are real mother of pearl then they are worth something in their own right.
smkummer:
A factory letter will give you the specification of the gun when it left the factory, the date it was shipped and where or whom it was shipped to. if a special order, a custom piece such as a registered magnum or a presentation gun for a famous person, then the letter will add value and prove (or disprove) the stories behind it. For the average gun, not so much.
Lanyard rings were available from the factory beginning with the Military & Police Model of 1899. if the serial number is defaced by the lanyard then I doubt it is a factory part. if it is stamped to either side then it should be factory and was probably a special order.
4dragans:
Jacketed ammo takes longer to deform into the forcing cone and rifling than lead. This means the pressure spike is maintained for a longer period. In 999 out of 1000 guns I doubt there will be an issue. But why risk it? The load at the time was the 158 grain lead round nose. Any load that duplicates that or the 148 grain wad cutter load should be safe. As for how much powder to put in? I suggest you go to the reloading sub forum and ask there:
http://www.thehighroad.org/forumdisplay.php?f=15
Its not a matter of how much powder, so much as which powder, how much of it, which bullet weight, what overall length, whether it is crimped, etc.
I do not reload, nor will I give advice on how to do so.