On the 4 speed on my 68 FHC the fill plug is behind a 3 inch diameter rubber plug on the left side of the transmission tunnel pretty much right below the the left side radio speaker on the console. I cannot tell from the photo if you have removed the same rubber plug. I do not know for certain if there are changes to later cars or the 2+2 but I have always thought the configuration of the Jaguar 4 speed gear box remained the same through out production.
David Barnes
68 E-Type FHC