Honda-SOHC

You could buy the cheapest and fit the cover onto your base.

I'm afraid I don't quite understand. Do you suggest to order a new repro seat, then remove its cover to cover my old one? But the problem remains. It's about the optics. I'd like to have that (semi-) chrome strip around the seat again. I've heard that unlike the older CB500 model, you can't do it on a CB500K2-ED which model has the same seat as the CB550, CB550K1 and CB55OK2. I'm told that strip is vulcanised on the seat.

Hmm... some have warned me that for my particular seat (77200-374-003) a new cover may not be a good option. Reason is that on this seat, that also was on the US models CB550, CB550K1 and CB550K2, you will never get that chrome strip back on again.
So what's the quality of this repro and do you know where it is made? https://www.davidsilverspares.co.uk/parts/by-part-number/partnumber_77200374003P/
Is it the same as this one: https://www.cmsnl.com/products/seat-compdouble-non-oe_77200374003p/#.XjXvyiPvK_g
And how about this one? https://www.ebay.it/itm/HONDA-SELLA-PER-CB500-FOUR-K2/143312783920?hash=item215e1b8a30:g:tn8AAOSwpmhbwty2

Like you I look forward to see the result. The dilemma I have: the original steel seat body on mine is just too good to replace. BTW, the gaffa tape on the document lid is not a bandage. It is there for when I need to repair something. For long rides like on vacations I can recommend an optional woolen seat cover like the one I cut out of an abandoned carpet. It is very, very comfy. Unfortunately my attempt to dye it black has failed miserably. Suggestions anyone to get it real black?

Thanks Bryan. Does such a cover come with instructions? So far I have not found vids that demonstrate how to do it.

I consider ordering this exhaust bracket from CMSNL: https://www.cmsnl.com/honda-cb500k1-four-european-direct-sales_model14661/brkt-bl-muffler_18430323040/#.XiQImCPvK_g Looking at the number, you'd say it must be original, but the picture shows a bracket that is a bit unlike the OEM I have. The one in the pic is a bit round where my old OEM one is slightly more edgy. Now I'm confident the bracket in the pic will be compatible with the newly reproduced CB500 mufflers, but will it also fit my old OEM exhaust? More in general what is the policy in denoting original parts numbers with aftermarket parts at DS and CMSNL?

Yes, for the CB550F models they are supposed to be 069A. See pic where that number is stamped in.

Middle (3^rd) position was standard with the airscrews out 1 turn ± ¹/₈. According to an in 1977 (!) edited booklet by American Honda Co. Ltd, US had 4^th groove from top and the airscrews 2 turns out. I don't know what brassware Gerben uses: OEM or aftermarket.