GTA Cambelt, 20mm or 29mm

Hi I am up for a new cambelt in then next couple of months, I read on a forum maybe this one (although can't find it) or perhaps another, regarding the replacement cambelt for a GTA being available in a 29 mm width instead of the standard 20 mm one.

Does anyone know/heard of this ?

This can't be right. Even my 30 year old Lancia Twin cam engine has a puny 20mm belt.

The old 164 24V unit has a 29mm belt. I'm almost completely certain the later 24V engines are the same.

I maybe wrong, happy to stand corrected . .