Yep got a brand new , no 12 here that measures 305 compared with the list at 295 and a no 6 that measures 345 compared with the list at 341

i think production tolerances come into play here . IT might be sensible to give some latitude to the quoted numbers.

Of course the makers can control the gauge of wire that they use , so the listed size for that should be pretty close ( to give wire sizes down to 0.01mm seams a bit daft though ), the no of coils would be within their control too.

They would probably be affected by manufacturing tolerances on OD , ID and length, although i agree it's curious how they come to od vs id numbers that don't correspond to 2 x wire thickness .....

I cant answer that and It's a bit of a fudge to say that the figures should be taken as approximate. That is how i use them .

These are not my figures . I found them on the listing . I have been using them as approx rather than absolutely perfect , and they have served me adequately so far.

The main reason for posting these numbers was to highlight the error about the no 12 dimensions in the original list .
I had "spoken " to 2 experienced tuners who were both ignoring the 12 because they were using the incorrect numbers in the original table .