SERIES ONE PRODUCTS:
700 production began in February 1971.The earliest production 700's had the serial number stamped on the transformer end bell. See 700 #R053. There are no records of how many like this were built but it wasn't many. Production in the very early days was only a few amps a week. R053 is one of the highest end bell serial numbers I recall seeing. After that a four digit sequential number system was used (the highest I've seen is #0121). Again, production figures were low. In December 1971, with the start of the Model 400, a date coded serial number system was introduced. See the factory serial number bulletin Front and Back. Note: this list was for warranty use only and doesn't include early 1972 units which were out of warranty by then. They had date coded serial numbers too.
400 production began in December 1971 with date coded serial numbers.
Both the 700B amplifer and 4000 preamp were introduced in July, 1973. Since 4 had been used to designate the model 400, 5 was used for the 4000 preamp. The first being #753-001.
Date coded serial numbers ended in May 1974. The 700B and 400 amps started consecutive numbering begining with #1000. The 4000 began with consecutive numbering in January, 1974 at #100 - I'm not sure why it was different.
See the factory serial number bulletin Front for other Series One products.
The 5000 FM tuner and Andromeda III speakers (later re-named Phase III) started at #1000 too.

SERIES TWO PRODUCTS:
Series two products which were a continuation of Series One models start where the Series One's left off. Series Two products were not all introduced at the same time. The 6000 Audio Delay was the first product with Series Two styling followed by the D-500. See photo of the Prototype 6000.

First Series Two Serial numbers:
200 Series Two: #6000
400 Series Two: #20000
700 Series Two: #7000 Note: early 700 Series Two's had leftover 700B labels.
2000 Series Two: #8000
4000 Series Two: #14000
5000 Series Two: exact number unknown. Probably in the mid 2000's.

PIONEER BUILT PRODUCTS:
Pioneer had their own serial number system different from Phase Linear. Pioneer built products (5100, 7000, 8000, 8000A, P510, P530, P560 and P580) serial numbers consisted of two letters followed by a long 6 or 7 digit number.

PROFESSIONAL SERIES PRODUCTS:
The Professional series was introduced in 1980. All Professional Series products started with #1000.

DRS SERIES PRODUCTS (Series Three):
The DRS Series was introduced in early 1982. All DRS Series products started with #1000.

FINAL NOTE:
A "B" stamped after a Series Two serial number indicated "B stock". These were freight damaged, warranty replacements, or demo's marked as B stock so they couldn't be sold as new. They were typically sold to employees at a reduced price.