I used to have that same Bonetto Cinturini strap Model 324 (and a Squale Maxi, too).
Replacement Watch Band Rubber Caoutchouc - - Bonetto cinturini
I believe that strap is only available in 22mm so it's only natural that you had to notch it down to 20mm to fit the Maxi.
The pattern is a weave resembling carbon fiber appearance. Definitely not a Tropic style rubber strap.