Ah - I understand your measurement images now! I have just had a look at some M42/T adapters that I have got and the thickens at that point is possibly half a mm. Nowhere near as thick as the one you have shown. One of mine is a little thicker, perhaps 2-3mm, but it has a section cut out to be thinner by the locating locating mark to accommodate the lever.
Looks like the adapter is faulty or marked for the wrong mount.