Each batch is one month so the number is really a "month number". When they got to 9900, they rolled over to 0000. Naturally this isn't a very good system which is probably why they introduced the 6-digit serial numbers shortly after.
Cool, cool... how do u know so much bout these things? I'm trying to sell it. Noticed couple 1978s Phaser PH-1 went for about $150-ish on Ebay. Just plunk it on and see what happens but prolly not worth as much as 1978?
BTW great forum and noticed BOSEAREA you're from the UK. I lived there for 20 years Glasgow - Insanely good beer.
A lot of very good information has come up on the forum over the last few years and there's some very knowledgable people here that are regular posters. Spend some time here and you can't help learning a lot about these pedals
I'm in the south of England so it's far from Glasgow but the beer is still very nice