The ENV differiental is easy to spot; it has no inspection cover. If you see an inspection cover you have either a type 27 or a type 30.

All Danas prior to 1967 were Model 27. Beginning in mid-production of '67 models, the 122S received the Dana 30. I suppose that if your car is toward the end of the production run, you might have the Dana 30.

I don't know what the differences between the 27 and 30 are. I think the type 30 has larger diameter axle tubes but I don't know what sets it apart from the 27 otherwise.

I think the type number is stamped on the right side of the pumpkin, below the centerline of the axle, on the back side.
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Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL