Re: Pickup cover sizes?
The U.S.A. standard spacing (which Duncan use) is 49.2 mm, while wide spaced is 52.4 mm. Since millimeters are so short, the fact that the spacings aren't whole millimeters seems a bit silly. However, it's because they are actually after-the-fact metric equivalents of 1-15/16 inches and 2-1/16 inches, which were the units used when those spacings were initially designed. Pickups using these spacings are, in reality, still built to the same "old" spacings of 1-15/16 inches and 2-1/16 inches. But the specs are now expressed in millimeters (usually), since the metric system is a more widespread standard these days.
The metric versions of standard and wide spaced are 50 mm and 53 mm. These sizes were designed from the start using the metric system, so they sensibly use whole millimeters.
There is enough of a difference between parts spec'd at 53 mm and parts spec'd at 52.4 mm to make parts non-interchangeable IME. It's the same deal between 49.2 mm and 50mm. My 50 mm covers don't fit my 49.2 mm pickups, and 52.4 mm covers don't fit the 53 mm bobbins that I have.
Duncan make both 49.2 and 52.4 humbuckers. A neck pickup will always be 49.2, unless it was special ordered otherwise. A bridge pickup could be either one.