I have a '39 Plym convertible coup that has single springs on both sides of the top. The '39 Plym's were the first factory open car to have a power top, it is vacuum operated.
During the course of restoring my Plym I found that the top assembly was very similar to the Fords, which made sense because both Ford and Chrysler built convertibles of the 1930's were built utilizing Briggs bodies.
Rebuilding the top assembly for my car was quite a challenge, had to have a lot of the parts made, the return springs were just one of the items.
Turned out there is a company here in Ventura that makes coil springs for all types of applications. I had to have twelve sets made, that being the minimum number their production system could make.
Post a pix of your spring, with the size, I'll compare to what I have. If my springs won't work on your car, send me your broken spring, I'll get a quote to reproduce it. of course they will have a minimum of springs, there are a lot of 46-48 Ford Conv's out there that could use new springs.
Pix attached of new 39 Plym top springs.