Firstly, this is now becoming a new thread.
Whilst I certainly don't claim to be aware of everything dubious to do with remarking/marking glass I do try to keep abreast of these things. I have never heard of this particular mark being reproduced before now.
When you received the offer Frank, did you see a finished version of the Rembrandt Guild mark - or was it just an unlikely boast made by the person who made the offer?
There have been a number of trademarks that I have seen over the years that have claimed to be fulproof, but of course they were quite detectable in the cold light of day. These include Venini and Keith Murray.
I have a strong suspicision that the Rembrandt mark would have been singularly unsuccessful had you taken this person up on their offer. It is after all, both complex and small. I doubt very much whether it would have been undetectable, particularly with the technology available in those days.
A simple, and usually successful (but not of course fulproof), guide to whether or not you are buying an item with a fake mark is its cost. If an item from the twentieth century is cheap for what it is, then IN ALL PROBABILITY it is unlikely to have a fake mark - after all you even allude to the fact that you could have charged a larger amount for your piece. It would not be viable to have it marked and then sell it cheap.
The majority of the pieces that I have seen with the mark have been eminently buyable. The ones that have not been cheap, I was aware of their 'pronenance'.
Since you did not take up the offer and IF you didn't see another piece that had this particular fake mark placed on it's base, then I feel it is a little rash of you to suggest that many Rembrandt pieces could bear fake makes.