need some 1665 crystal help

[PR78]

Was hoping someone can help me out here:

im trying to get a superdome on my 1665 which currently has the lower "service" dome on it.

i bought a $30 dollar one on ebay but it didn't fit, the crystal was too big. i've since returned the ebay dome but now im pretty confused because the ebay auctions say 30.25mm but my experience suggests that it should be something like 30.19mm

can someone clarify what the correct diameter of the 1665 crystal is?

thanks.