The door or lid as I can it is for the owners manual, no idea why they call it the service book as that's a book the dealer used to stamp when you had the bike serviced. We had boxes of those at Queenies, they don't come with the bike, the owners manual does.
However that bag might be right these days but back when the 500 was new the bag the owners manual came in was black and had a press stud on the rear. It came in a bag otherwise the book got wet when the seat foam held water due to rain etc and when sat on by the rider the water would be squeezed out by the holes in the seat pan, even though there were no holes were the lid was it still got wet somehow so the manual needed some protection. Any zip lock bag would do the job as good and far cheaper.
Handbooks are still around but dear when you can find one, especially in good nick. I sold all mine a few years ago thinking I was breaking the bike, I now have a 550 one instead as all the electrics on mine are 550 anyway and TBH it's the only thing I may need from the handbook anyway.
Download of the original CB500 Owners manual here.
https://www.honda4fun.com/dwnload/Owners-Manual/CB500-K1-Owners-Manual.pdf
