Lee-
Are you talking about the backstay chainplate, the reenforcement of the transom, or the actual backstay?
Quent

Lee-
If I remember correctly, there are several approaches expressed re backstay chainplates and transom reenforcement. Mine was to add a large but tapered patch of fiberglass inside the transom, including the inward flange at the hull/deck connection which greatly stiffed the transom center. The replacement chainplate was (I think) 3/8 by 2″ stainless bar stock. I used 316, but would probably use one of the duplex alloys in retrospect because 316 is less strong that 302/304, and no more resistant to stress corrosion cracking.
We have a sloop with a single backstay. Ketch rig, or if you opt for double or split backstay, your decision may well differ.
Quent

Lee
The repair only applies to sloops and cutters. I’m not sure how many 424s were made with that rig. Here’s a link to my project pages. Click on the transom chainplate tab to view my notes. The project would work better if you welded the inside bracket assembly rather than bolt it. I had problems with the bolts interfering with each other. I also suggest electro-polishing the chainplate before installation.

There is absolutely no sign of stess on the chainplate anymore.

http://www.richardcarter.net/Projects.html