68 GMC 4X4 rear axle

[Chazbo]

Having a difficult time in determining what type of axle I have .....the rear cover is 10 bolt and about 12" in diameter, floating axle with a 10" ring gear (maybe 10.125")....the differential come out as one piece from the front with same 10 bolt pattern.....has a 4:57 ratio....GM corporate as stamped on the gear and parts....has an "M" stamped on the diff housing and appears to have clutches (limited slip)....someone said that it may be military...and someone said it may be a NAPCO rear end....some say it is actually a 1.5 ton rear axle.....I can supply pics if needed