It's rather obscure then and now, but it's a saltire, representing good luck or good omen. Although it is associated with St Andrew, it is not exclusively, so is likely, but not certainly, Scottish related.
I have other ARP sweetheart-type lapels that have lucky horseshoes below the badge. Obviously, the idea was to bring good luck/protect the wearer from being blown up whilst on duty!
See for more details about the saltire's meaning:
https://www.seiyaku.com/customs/crosses/andrew.html