When I purchased the spring from Midway the technical information is as follows.

Factory replacement part from Smith & Wesson.

Technical Information

Features: Smith & Wesson factory part

Yes it is a more robust spring than the original but it was recommended for all the autos listed below.

1006 Auto 1026 Auto 4505 Auto 4506 Auto 4526 Auto 4546 Auto 645 Auto 745 Auto

Now what was interesting is I did not expect this spring to work with reduced loads yet, it was not an issue.

As far as the visible spring difference between light and heavy there are less coils on the light spring.

Having seen the top of a stainless frame peened by the barrel contact I would rather have the heavy spring than a lighter one to prevent frame wear.

Bruce