These were the GEC type designations for the their 'Red Spot' range of fuselinks. The first letter was a 'family' name for fuses with a similar purpose, the second and third letters I'm not at all sure about, if they ever had anything other than an arbitrary meaning as a code letter.
This catalogue from Lawson has a fairly comprehensive cross-reference for fuses manufactured by GEC, Brush, Dorman and one or two others in the back.
Thanks for that Scotty. This came up in a conversation with a colleague. I kind of suspected that the letters might be just a manufacturers reference but wasn't sure.
Well the T series are general-purpose 415V AC / 250V DC fuselinks, the SS series are similar but 660 or 690V rated (?), the J series are fuses with modified blade arrangements for use by the utility companies, and after that I'd be guessing!
