My dad has a old (1935) Marlin in .38-56. Collectible or shooter?

Never heard of it, so...
"collectible".

I think you mean 38-55.  I don't know if they are worth that much.  Supposed to be a good round.

Nope, it's a Marlin 1895 in.38-56 Winchester caliber. Heavy bastard with an octagon barrel. My dad made ammo out of reformed .45-70 brass but hasn't fired it in about 20 years.

I stand corrected, never heard of one before.

The .38-56 Winchester Center Fire cartridge was introduced in 1887 by Winchester for the Winchester Model 1886,[3] and was also used in the Marlin Model of 1895. Production of Model 1886s chambered in this cartridge ceased in 1910 due to lack of demand,[3] while most commercial production of the cartridge itself ceased in the 1930s. New production loaded cartridges and unloaded brass cases are rare, and are often created using reformed .45-70 brass. The cartridge was originally intended to outperform the similar .38-55 Winchester, but in reality had very similar ballistics despite using more gunpowder

Dont feel bad, I had to google it as well.

I'd shoot it.