Ach, I'm stupido again! Upon careful re-read, it's the cartridge retaining "pip" on the BACK of the charger, that's being of interest. There's a pic floating about, but I'm not smart enough to paste it directly. So another link:
Charger queston - Page 2 - Military Surplus Collectors Forums
Post #12 shows the idea, I hope!
Sorry, Enfieldlock! Feel free to slap me upside the head.