For T91 Mil-Spec barrel, its has 15mm (0.59") gas port landing.
For Wolf A1 commercial barrel, its has 18mm (0.708") gas port landing.
Earlier this summer we've got some special ordered Mil-Spec and commercial style gas blocks which was modified to 0.625" (AR-15 Pencil profile, 15.875mm) and 0.75" (AR-15 Standard, 19.05mm) to fit U.S. AR-15 barrels, but it's no longer available (For special order only).
To address T91 owner's needs in the U.S., we realized it will add more benefits to T91 owners if the gas block can be installed on popular AR-15 Standard barrel which is 0.75" (19.05mm) gas port landing also make it easy to swap out any barrel anytime.
If we received more demand on 15.875mm (0.625") gas block for pencil profile barrel, we may special order it as well.