I think the Mk 6 still has the same length cylinder as earlier models. It was made from part British and partly Spanish components but as I don't own one, I may be wrong on the dimensions. Certainly and as you say the wholly British made Mk1 to 5 Meteors shared the same cylinder dimensions. The Mark 7 was a completely different rifle with a longer cylinder - IMHO the only element it shared with earlier Meteors was the name
John