A pair of rimless and rimmed FS Brodies, interesting stamps, and colour question

[Rabaul]

Hi All,
I thought members might find this interesting. I know there have been several posts about WWI British helmet ‘heat stamps’ and relative dating.
I have two ‘Brodies’ one is rimless, although in poor shape there is no sign of a ‘tide mark’ in the paint and the other is a fair conditioned rimmed one.
What is interesting is they are both FS (T. Firth & Sons Ltd of Sheffield, I assume) marked and the numbers are only 4 digits apart FS143 and FS147. However the rimless has a raise F and lowered S and the rimmed has a straight FS.
I have seen somewhere that the alignment of the maker’s initials may have a bearing on dates. Is this true? I’d like to hear any comments.
Also I have included a close-up of the rimless ones paint remnants. The base colour appears to be pale grey, and then it has a light khaki, and then a horrible thick top coat of post-war ‘park bench’ gloss green.
Is the grey the fabled first grey colour which is often mentioned, which was issued along with the early apple green?
Regards
Steve