OK, what you show is pretty much what I figured out on my own of what you have, which appears to be a cobbled together exhaust riser made from individual pieces of either black steel or galvanized pipe. It appears to be home made, rather than a purchased piece from C or CD. But mebbe not, I'm not there to check the material.
What you bought from Westerbeke is a pretty good find on your part, not many folks know about it.
What your real challenge is that you have the old entire assembly which was built to match the physical constrains of the space between the back of your exhaust manifold and the inlet of your muffler. And what you bought does not fit into that geometry at all, because it is not an exact replacement for any individual part or piece of what you have removed for replacement.
Your old assembly has the inlet nipple on the TOP of the horizontal part of the assembly. The new WB piece will have it on the vertical piece.
Because you are the ONLY person in the entire world who is anywhere near being on your boat, you will have to figure out the geometry of any and all additional pieces that need to be added to the new WB piece to make it fit and connect the manifold and the muffler.
For credibility's sake, I, too, have an M25 and have replaced three risers, so I know.
The other option is to return the WB piece or sell it, and ONLY IF you have a standard C30 PHYSICAL arrangement between your engine and the muffler, buy one from Catalina Direct.
Exhaust System
They seem to have four different ones, so no matter what you do, you're in for some serious measuring.
Good luck.