I just picked up a muncie 4 speed with a main case #3851325 With no rings on the input.

I was told this is a rare unit. Is this true and what value would it have.

not really rare, prob has a 7/8 countershaft. Good working units will go 350-600 used.

I sell 325's on ebay all the time.... they bring around 450-550 depending on condition and if they have a shifter etc.