There are two distinct types of the larger-sized ARP button - one with a "dish" (concave) rear and the other has a convex profile. The manufacturing method also looks to be different as the dish buttons have a prominent shine to them. The buttons on the central opening of my original bluette overalls have the "dish" style and I assume all loose dish ARP buttons come from the overalls (the examples I have all appear to have been made by Firmin). The convex style of buttons I assume were sewn to ARP Pattern 42 (Ladies' Warden Coat), Pattern 43 ambulance coat and ARP Pattern 71 tunic as well as greatcoats.
Note: only the larger size buttons have this difference. All smaller sized ARP buttons appear to have the same style (convex).
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