Re: Marlin 1893 38/55
You are indeed fortunate to acquire your great uncle's and his grandfathers 1893 Marlin. Looking at your pictures, it looks to be a classical 1893 with case colored frame and much of the original finish in pretty nice condition.
By all means, keep the entire gun original. Resist the urge to touch up the wood or metal finish. Take it out and shoot it once in a while, give her a good cleaning and wipe down and consider your self lucky.
I have some older .38-55 cartridges and boxes and yes, those are 255 grain bullets.
I would guess the date it left the Marlin factory was around the late 1910's to 1920 or so. And My guess as to value would be in the $1500 to 2000 range.
Hope this helps and hopefully some more responses will be added.