British helmets came in 6 basic designs during the war. The Mark I which was your basic "Tommy Atkins" tin pot. Which was very similar to the tin pots of WWI. This was improved to the Mark II and issued to the troops as early as 1940. It reduced the rim fringe area and was thought to give better overall protection. There were two different patterns for this helmet. Then came the Mark III "turtle" helmet which was first issued to the 3rd Divison for their assault of Sword Beach. The Motorcyclists, Tankers and Airborne units came up each with rimless helmets as they decided that the Mark II or Mark III were not good for them. These three helmets were the same shape and only differed with the interiors from each other.
This can get quite confusing, but basically the infantry tin pots reduced the rim and became more round in shape. The armor, airborne and motorcycle troops decided on the same rimless pattern with different straps and other attachments. So you can use an airborne helmet on a tanker and vice versa.
Just view your references as to the units and the proper helmets. Some units were given a helmet type ahead of general issue.
I hope this helps.
Mike T.
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