Generally the second # does stand for the Number of grains of black powder , but is sometimes a bit off. I recall reading that a 38-55 would hold nearly 55 gr in the case with no bullet, and although I dont think that original designer Ballard meant it to be used as a breech seated round only, it was used as such for target shooting and without bullet inside the case, they may have been able to cram almost 55 gr in ? (although unlikely unless really crammed somehow)

Some designations were rigged a bit to have a  perceived better marketing designation, with an example being the “38-40” which would indicate 38 cal with 40 gr of b.p. .In reality it should have been designated 40-37 to accurately state load , with a true 40 cal (.400 -.401 ) bullet and 37 gr B.P. Originally designated 38 WCF (instead of the true ’40’ ) by Winchester its developer, by necking down their 44 WCF.(true dia size being.427, so accurate designation never being a foremost concern ) I think the 38-40 designation was originated by Marlin ( they did the same renaming of other Win Cartridges ) who wanted to use the cartridge in their line -up , but not give credit to the Winchester company

Phil