I posted this over at the XK8/XKR forum, but still no answers. Since the gearbox is common between many Jag models, perhaps I'll be more successful here.

Is this a fill plug? I have the 6HP26 gearbox, I thought the fill plug was on the right side...

 

Hi Azeteg,
Try this forum post on mjaguars, hope it helps, I usually do not look for info on other sites but here goes:
ZF transmission oil and sleeve change writeup with pics FAQ - Page 8 - Jaguar Forums - Jaguar Enthusiasts Forum

Good Luck, there's some images that may help too?
Regards, Stu