Stock rear end?

[svr]

I Traded a 2 ton all steel flat bed and some cash for a 57, 3100, short box basket case. It is in primer and most body work has been done, steel is pretty sound. It started life as a navy pickup then went to a local school district for many yrs. Anyway trying to make a plan, Going to leave the 235, it seems to run fair enough to get started. Going to swap out the 3sp for and s10 5sp with .72 over drive so I figure i need a rear end in the 3.90 to 4.10 range and thats what the stock gears should be. Question, inspecting the rear end I found a small hole maybe 1/8" or so in the top of the left axle tube. It appears to be an open hole, was there a vent of some sort there? Or has somebody done this for what ever reason. Thanks