The problem with all this is toofold. Firstly Stamps can be made no problem- you can order them from a reputable chasing tool company in the States.
Secondly the Stamps can create confusion about whether a disc is genuine or not. On aprevious thread there was questions around whether or not the Yum Yums had stamp in it and somesaid that all ABC demos had it when infact many did not - Come on by jay Traynor would be a good example. So the point is Just because it doesnt have a stamp is it genuine?
The answer is it very well could be depending on the pressing plant used as some majors appranlty used small local plants(often used to rush small quantities) who did not use the stamps. This is still the case today according to Peter Brown who I met last year in New York and does it for a living.
So overall unless you have some writen confirmation about where it was pressed /date etc otherwise known as providence then you cannot trust anything! even with 30 years or so in the business.