You need to research that further, with EAA. The interpretation is that
if
you hold a single engine land SP, you are good to go on all sp SEL. if
you want to fly a seaplane, you can, if you get a signoff for that one
model. Same for a twin.

It closes the loophole that has allowed a SEL PP fly an experimental
seaplane or twin, with no checkouts or controls. No longer, with a SP.


I'll look later when I have more time buttttt. I think I'm right.
A SP from scratch gets endorsed for his make and model and must be
checked/tested for additionals. I may be wrong (I thought I was wrong once,
but I was mistaken!).
John