Good stuff never seen a C64 in the UK must keep an eye out for that size
Yepp, rare as the proverbial hens teeth. As we all know, the number of people with a specific size are distributed along a bell curve (although the bell keeps moving in the larger direction as the population is getting bigger and more overweight), so C64 was rare even when the snow suits were issued. There were a lot less guys with 128 cm chests in 1962 (at least in Sweden), and the smocks are getting old, so only a small percentage of those made, if any, of that size are available on the market.
I have an excellent book from the Swedish master tailors guild from 1957 with cutting drafts for all uniforms, military and civilian, that were used at that time. Unfortunately it predates the snow smocks with a few years. Otherwise you could have gotten a draft.
Here is the size tables (which are just standard sizes really). The numbers indicate metric
body measure that is without ease, just taken with a tape over the body (or underweear depending on who you ask).
http://www.beredskapsboden.se/sv/Storlekstabell.html Scroll down to "herrstorlekar", the different rows of numbers refers to the numbers in the picture in the following sequence: 1, 5, 2, 3, 4.
Big EDIT: The snow smock does not follow the same convention as the rest of the uniform system! C54 is made to fit C54-C58, so it might fit the OP as well. C50 is made to fit C48,C50,C52,C146,C150 and C152. Sorry for the confusion. At least I learnt something today.