For an '01 Bullitt, the 3L27 is correct, as they all came with 3.27s. The next letters should read 88 for an 8.8" rear, as you say. I think you're right, it's just a misprint.
Yeah, so bizarre. I was reading elsewhere that ford did indeed make an 8.5 rear end back in the 70s that they used on some LTDs. I would find it very surprising if one of those found its way underneath my car, especially with how readily available the 8.8 rear ends are.
The previous owner stated that he had the rear end rebuilt at one point...maybe the shop grabbed the wrong tag. But then again, the date code on that tag indicates April of 2001, which correctly corresponds with the bullitt build date. And I don't think ford was making any 8.5 rear ends in 01.
A mystery.
Is there anyway is verify it's a real 8.8 without removing the diff cover?