If you own one of these beasts, you'll know that getting a strap can be an issue, with the 21mm lugs. Also, the spring bar sits low in the lugs, which can result in a deep gap between the strap and top of the case.

Vintage 21mm straps are bloomin' expensive, and hard to come by...

I've recently squeezed in a 22mm Isofrane 'copy' and it's perfect. The rubber is seriously good quality, and very thick. It has slightly bulged ends that fill that lug space nicely (it's 7mm thick at the lug end), and has the tool-like vintage look.

I got mine on ebay, but you can get them new from Time Factors, who host the TZ-UK watch forum that I frequent.

The design has a 'style line' groove that runs down either side of the strap, I reversed mine so that it's plain, to give a more vintage rubber look, and it's great.

Anyway, I thought I'd share my experience, and at £15 a pop, excellent value..

Second strap down on this link..
http://www.timefactors.com/rubber.htm

(PS. I've no vested interest in the business)

Cheers
Stewart