Marzocchi info is hard to find Jerry. I have a pair of piggy back MX360 Marzocchi shocks that I can't find seals for. I haven't dissambled them and tried around bearing places and the like yet....
Maybe the GA cast is a clue to the forks being from a Maico? I think their 77 enduro models had the GA prefix....not %100 on that though.
As for advice, I wouldn't be drilling any extra holes OR fitting those PDS valves until you have ridden with the forks the way they are. They were apparently a very good fork.
I put PDS valves in my Betor forks on my B50 and it was a step backwards. And it cost me a set of damper rods to find out as you have to make and sweat a socket to fit the bloody things
Luckily I was able to get another set of Betor damper rods..... Cost me more money though.