Flektogon 20mm f2.8 thread size question

[overflow]

I tried two 67mm UV filters on my Flektogon 20mm f2.8 and none screw despite the Flektogon should be 67mm, in fact they're a bit loose when trying them inside so I reckon it may be one or two mm bigger the screw. How is this possible? thought all Flektogons were the same size. Any ideas?



Thanks

[visualopsins]

My (not Flek) 20mm needs 72mm hood to avoid vignette.

Hope someone here can help!

[padam]

My 20 2.8 Flektogon came with a 67mm Tamron UV filter which fits perfectly, but it also causes a bit of a vignette at the edges, so I just removed it.