The D350 and D300 were like the D400, with a few differences. I am not sure of them, but I am sure DA-Aquamaster can explain them. These regs were some of the few great revolutions of regulators. They breathe like a dream, but a little complicated and finicky.
The D300/350 use the same service kits as the D400, which are still available. The D400 was packaged with both, so I am not sure which first stage you have. You can either find service kits on ebay, or through a contact at the dive shop.
Edit: Found this in another thread from the SP lord on the board, DA Aquamaster:
The D300 was the only model that used a solid one piece light blue dive/predive switch.
The D350 introduced the black switch with the D350 sticker. These stickers can be found in both pink and blue as it was intended for the customer to use one or the other depending on whether the user was male or female.
The D400 looks almost exactly like the D350 and if someone puts a D400 sticker on a D350 it's usually not possible to tell them apart until you remove the purge cover and diaphragm and discover the D350 lever. The case changed about half was through D350 production so later D350s have the same case as the D400 while early D350's have D300 cases - the two cases won't look any different to the casual observer, but they won't interchange.
As far as I can tell, you have a D300