Ok I know it’s been discussed several times and I know that –
SGS is Long
GS is medium
(Or long and extra long depending on who you talk to)

What is the cage length (between centre of jockey wheels) for each?

I have here what was ordered as a GS and it’s 80mm but that is the same as the one I already had for last 2 years which I bought as a long cage

Sheldon is further confusing me as that says 74/86mm

Original one came from bikediscount.de
This one has come from Ribble

Trying to find out which one was advertised/sent incorrectly!

I have just measured my XT Shadow + SGS and it is approx 95mm between centres taken very roughly

Yep looking at this
http://si.shimano.com/php/download.php?file=pdf/ev/EV-RD-M786-3321.pdf
And the slight difference in shapes between the GS and SGS inner cages I have deduced that I think this is indeed a “medium” GS

I’ve just collected one of the GS XTs from Ribble at the weekend and I thought it looked a bit long too.
Only convinced myself that it wasn’t SGS when I put it alongside what I knew to be an SGS.