Seabee / Naval Beach Group (NBG) uniform

[Tonomachi]

I purchased this uniform for ten dollars at the flea market. It is dated 1968 however I didn't notice the missing right shoulder patch until I got it home. Does anyone know what would have been sewn on the right shoulder as whatever it was looks circular. What is interesting is the rectangular piece of blue cloth on the back of this uniform which I'm assuming was for others to be able to identify these specialists on the beach quickly.

[Tonomachi]

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