Hi Andy, cool indeed, thanks for sharing.
Yours is a true beauty, very bright chrono hands and so clean pusher and crown stems - where most 156's seem to be always so dirty over there, one could think verdigris was applied at the source :-)...
Your watch instead looks like new, out of a time capsule.

Interesting this use of L5100 and giving up on the 24 hr subdial, but most likely it was the easiest solution - like the date-only 510.50x's. And the same most likely happened for the Silverstones and the Cortinas.
The Sinn ad says same movement as the 140, which we know has been a very contentious topic ever since the (in)famous Furrer picture onboard the Space Shuttle - and the "Sinn 140 / 142" caseback inscriptions.
Actually Sinn put the record straight in their 50th anniversary jubilee catalogue, stating that Furrer's watch was a 140 which was later superseded by the 142.
Cheers,
Fabrizio