Any clue to engine size? XJ6 would only be 4.2 (or 2.8 but I don't think that was sold in US) - other 4.2 were 64+ E-type (or XK-E in US), 64+ MkX, 420 and 420G - e-type the ony one with 3 carbs as standard, but easily upgraded from lesser versions. 3.8 has many possibilities - 3 carbs were 61-64 E-type, Mk2 3.8, XK150 SE and there was also the S-type (which I forgot above) with fewer carbs. 3.4 gives Mk2, S-type, ordinary XK150 and various earlier XK, 2.4 only gives Mk2 in 60s. 1960-67 Mk2 3.8 would be my best guess at the moment for numbers produced.