Pics would help.

It could very well be the bike offered to you is a Cove example. I understand it reads 'Holland' instead of 'Amsterdam'. That is an indication. With my RIHs Amsterdam is mentioned on both frame and badge. 531 was used for Cove RIHs. I know there were 531 main triangle only frames, but possibly also full 531. With the build quility you can easily tell them apart. Compared to the originals (A'dam) the Cove's show 'crude' frameparts. The specs you mention sound more typical for a Cove RIH, than an A'dam one.

With my RIHs there is a 4 digit serial present on the lower headlug. On the Cove RIHs it is elsewhere and they use another system.

Cove acquired the rights on the RIH brand decades ago. They paid a price for it and draw benefit of it. Personally I am of the opinion they didn't contribute to the strong brand themselves. It were the original RIHs that were raced to 63 Olympic and World titles and it are the originals that show superior quality.