That base is actually the second tier hi arch moulding, released about 1977 to be positioned below the premium Mustang hi arch base, mainly as a store brand hi arch as well as some lower positioned Victa hi arch models. Mustang thumb latch catchers were slightly different at the meeting point with the base and actually won't attach properly to the second tier hi arch base, but the catchers off these bases will go on the Mustangs!
You will notice though, that the stepped profiles where they meet, don't match properly.
The funny thing is that later Mustang catchers from Powertorque series models are a perfect match with these second tier alloy bases, so go figure.
These bases gave the stores a chance to market a high arch model of their own and for Victa to offer a budget hi arch with less premium features such as nylon bearing wheels, plain shape non comfort grip handles and screw in handle knobs (instead of quick release levers to fold the handles) or any combination of these features -or non features!
Victa was just as marketing focused as engineering.
