Honda-SOHC

550 and 500 same shape but later ones had a connector bit, the screwed in retainer does not have a seal, only the big tim pressed plate

Exactly that's why I stayed with the original seperate units - Im chicken!

My 500 has eight rubber dampers four are thick on the drive side four are thinner on the overrun.
Looks like the 550 is different.
Definitely another seal on the retainer item 20 in the parts book p/n 90753-283-000 the left side spacer runs inside it plus a large O ring for the sprocket cover I think.

No matter what set you need thicker O rings at the headgasket than those in the set

I feel your pain, I've been waiting two weeks for spark plugs that were advertised as in-stock but so far nowhere to be seen!
Was the old sprocket worn out? I was lucky with Ratty because although the drive chain was seized solid the rear sprocket was perfect apart from the PO having it chrome plated! I had that removed and it was refinished in BZP and it has come out like new.

The old sprocket was worn, corroded and was in overall poor condition. I didn't want to use it with a new chain and front sprocket - rightly or (probably) wrongly I slung it. I'll have to wait and see if the other one due next week is ok. And yes the simplest little brackets and fasteners are like hens teeth.