After some research when I bought one of these last year from Summit Racing I learned that the primary difference between their thermoquads is the rebuilder and the core charge. The less expensive one is rebuilt by UREMCO for a specific application, however it will fit others. The more expensive one that you have is intended to replace any 73-77 thermoquad. Unfortunately Summit wouldn't tell me who rebuilds the more expensive one.
Both carbs are rebuilt thermoquads; one is not significantly different than the other. They tried to tell me that one was a performance carb, but when I inquired as to the specific difference, they offered explanations that didn't pan out when I called back to verify that information.
