The secondary piston stop is about .25" longer

Well, That is dramatic enough so it should be trimmed down. But if body length is identical along with seals and placement. That becomes an easy fix if you need the secondary piston. I assume then, the primary is a good match?

Here is a picture of what is going on. While hard to see both pistons are placed where the seals would sit just in front of the compensation ports. Leaving them open to atmospheric pressure of the reservoir. As the brakes are applied both pistons move and the seal closes the port. At this point pressure can build up equally in both chambers. The Secondary piston which controls rear drum brakes has a stroke of about 1/2" before bottoming on this 71up MC. The primary front disc piston has a stroke of about 1". The front piston overall length (some have adjustable spring lengths) determines start point of the secondary piston main seal in relationship to comp port. CAN"T cover comp port or pressure can not reduce when brakes off and springs rebound piston.

From a fellow member, I purchased a 2225601 master cylinder for my 1970 U-code Cuda . Does this Wagner rebuild kit (F104474) look correct for the 601 casting?

On the drum kit, sorry I couldn't respond to your last PM. I was in OH last weekend and working until late most evenings since I got back!
Here are pictures of pistons in an NOS kit I have for the drum MC. Not sure why the aftermarket kit has a retaining ring, the original MC's didn't have a retaining ring, the power brake versions used a retaining clip, and the non power had a ring with the dust boot.

Not a problem about the PM. Plus, maybe this information can help someone else in the future. I do have some more questions though. When you say NOS kit, are you referring to an NOS Bendix or NOS Mopar kit? Can you share those part numbers? From the pictures, there is a slight difference in length between the Wagner and NOS pistons. Plus the O-ring/cups are configured differently on one of the pistons. Do you see a problem with those differences?

Lastly, did I buy the correct kit? Does the Wagner part number F104474 equate to a 474 kit?

Thanks,
David

They do not look right.