This site suggests it is down to WW1 leaky battery acid: Scaleybacks

Academic
Question for SAS troopers, British Military and others:
I'm reading Seven Troop by Andy McNab, and a few times he has used the term "Scaley".
Clearly he is referring to some sort of military personnel apart from the SAS and SBS -- a mate named Nish explains he is not guilty of revealing his SAS role by virtue of having claimed to be a Scaley.
Online I see a few conflicting definitions, such as a married army member.
Anyone know what is meant in this book?
Thanks in advance.
This site suggests it is down to WW1 leaky battery acid: Scaleybacks
Thanks for that idea. I have some doubt about battery acid, but if they are Royal Signalers, that makes sense given the radio scale or dial.
The etymology might be dodgy, but they seem clear on who the term refers to.
BTW, is the book good?
It is quite good. McNab was billed as being "of the quality of Forsyth" (not quite); it's pretty interesting. I prefer his fiction though, having read 3 or 4 novels of his over the last month or two. There's nothing like the Day of the Jackal, however, for suspense, in my humble...