To the best of my knowledge the only hollows & rounds made for the #55 were eight blades for cutting hollows and rounds. They didn't have a special base for those. The central tower and shoe performed the support when needed for the round blades and other shapes.
The #45 & #55 have different spacings for the rods used to support the movable skate and fence and add on bases.
There are at least two of the nosing sets currently on the auction site. One is Buy It Now for $99.99 + free shipping. The other is $125, also including shipping.
If you know someone who wants to pay $1K they can have mine and you can get a 15% finders fee.
Didn't look deep in everything that popped up but there were a couple of H & R bases for sale without blades. Both were BIN @ $110 + free shipping. The blades from the #55 can be used with these.
For my needs, it was less expensive to put together an almost complete set (mostly even sizes) of wooden hollows & rounds.
Looking back to the beginning of this thread, that was my answer to, "if you could do just one premium plane." Though it is more like a few dozen planes purchased than just one.
jtk