What Diff?

Hi all
I picked up a gearbox for my project on the weekend, good old ebay. Its a mk2 GTI and the code is ACD.
It turns by hand nicely in every gear and shifts fine but I just noticed that the diff is locked.
When I turn one output flange the other should turn the other way, instead buth go together and turn the gearbox input shaft.
Was there a factory made LSD or locker diff? If there was, is there a way of identifying it without splitting the case?
If it was a LSD then I would still get some movement by hand wouldn't I?
I so far have not put any force on it just a quck try at turning the flanges in opposite directions by hand, but they feel solid.
Any Ideas?
Rob

Diff preload.

Showing my ignorance here but wosat?

There is a small amount of stiction built into the diff. This is what you are seeing.

Brill thanks.
I will fit it tonight with my mind at ease!