I'll find it when I find Castle Anthrax...

It's a Monty Python joke....
Seriously "Holy Grail" is a symbolic metaphor for something that you go through an arduous "quest" to obtain. Be it, working hard and having the discipline to save money to be able to afford one or diligently finding a used or grey dealer that sells the watch you desperately want for the price within your budget.
It need not be a PP. it can be any number of watches that are either extremely expensive or hard to obtain. Like for example AP, VC or even certain models of vintage/older Panerai.