I believe you are correct Mike. I think I pulled that post up, but the picture was no longer available. There is also a GunBroker listing with a picture of the U main spring. I did not want to link it since it is an active listing. The U spring was not in all Pre-Model 25's according to the pubication "Standard Catalog of Smith & Wesson". There was also a W shaped main spring.
I did not find anything discussing the advantage or disadvantage to the unique springs. I assume there are fewer revolvers with these type springs.