FAN-1 Wide-angle Converter,screw thread measure?

Hi!

I have a Fan-1 0,6x wide angle converter,http://www.ussrphoto.com/Wiki/default.asp?WikiCatID=28&ParentID=2&ContentID=1239&Item=FAN-1+Wide-angle+Converter

a beautiful heavy lens produced by ART-Filmotechnik,I found it used here in Italy for a very few bucks,
but the lens three step down ring was missing.
So I thought to measure the screw thread of the lens but it's an uncommon measure: 65mm ca.
And sadly there is no 65mm step down-up ring on ebay or any other shop,only 62mm or 67mm.

Have I measured right?is it really a 65mm thread?

I would like to thank in advance anyone who has any info to write about that...