Hi guys ( and gals - although I think we scared them all away? :weep: )

I know a ‘little bit’ about helmets …. but here is a question about a feature I don’t know - and maybe our experts out there may be able to answer it. Not earth shattering by any stretch, and doesn’t make or break the hobby,but interesting nonetheless I feel.

The helmet factories stamped their initials in capitals ( ET, SE, Q, NS & EF ) and during the war ET changed to ckl and SE changed to hkp ( codes to protect the identity of the factories )

1) why only ET & SE? was it the capacity of the factories … or their location, or vulnerability to attack?
2) Why are the "codes" in lower case ( there are some upper case CKL exceptions but not hkp )

…just wonderin’?
Dan