Just finished. A very accurate copy of the 'Smock, Windproof' 1940 pattern, aka the original 'SAS smock'.
I've changed a few things since the pic was taken yesterday, to match details of pics of originals, but those were small details like replacing the cords with cotton tape, some bar-tack on the pockets, and a missing stitch.
Other than that, it is very accurate but with a twist; this is made in Ventile L24. Being Ventile and having found Ventile doesn't actually breath so much, I opted for one single layer. The original had a double layer on the shoulders/chest and upper back, and on the lower front torso.
I wanted an accurate copy. If I wanted a modern, very functional smock, I would change a couple of things. Deep zip, 1/3 down the chest (this one has no zip at all, like the original version). Side vent zips at the armpits. And the lower pockets a little further down or further up so they would not clash with the lumbar belt of a modern rucksack. Possibly a touch longer too.
Other than that, it is a pretty good functional smock, and no wonder the modern versions are basically very similar but with a full zip.
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