I'm going to guess that your #490 was finish machined about the time Marlin was getting the last of the Brown receivers they inherited used up and were doing their own, since the barrel threads appear to be in the typical Marlin pattern.  That is the only use I have ever heard of where the reversible firing pin feature appears in the same block with a big old Brown hammer, so yours does still have some very transitional features.  It looks like Marlin was being very frugal with their start-up of their Ballard production... wondering no doubt whether they would be successful after so many others had not.  Regardless, you have a very neat find!

Froggie