M35 reissued helmet with number

[Jørn Schmidt]

I just picked this reissued M35 helmet up the other day,sadly its in a bad condition,the shell inside is marked 53,can anyone here tell me what this number cut mean,the shell
size is ET64,so I dont think it cut be the helmet size,I have never seen a helmet marked this way before.
Regards
J.S

[Jørn Schmidt]

liner & shell marking