The extensions were 7/8" for the G15CSR and GMk2 (same as 1960-62 G12CSR), and about 2" for the G15CS/N15CS.
Triumph2: I believe you have a genuine G15CS stand. From 1962 up, the G80CS came with long travel forks and the factory fitted 1-3/4" extensions according to AMC-expert Don Madden.
For the G15CSR & Mk2 models, an extension of 7/8" is insufficient to clear the rear wheel off the ground. This has probably as much to do with the extension size as the fact that the stand is completely inadequate for these heavy bikes and deforms easily.
Correct stands are depicted here:
Broken link removed
Please note an improved stand made in the UK has been available since last year. The stand has received a lot of praise. Fitted with the correct extension, it will positively lift the rear wheel off the ground. Send me a PM and I will forward details.
-Knut