The type of TBV that was used on the TR250, the 69TR6 and the 74 TR6 is like the one you have..The sensing port for the vacuum that makes the TBV function is a hole down at the bottom of that nipple.
On the rest of the years the sensing port is internal and must remain open if you want the TBV to function,
BUT. the small hole gets covered in the 250, 69 and 74 versions
This 69 TR6 illustration is exactly the same as the TR250
I'd disable the thing entirely if I were you...make a
substantial gasket,one capable of standing up to high manifold vacuum..in excess of 25 in-Hg during engine braking..a gasket that covers all but the 3 threaded screw holes that attach the TBV to the body of the carb.