via this auction
Details:
"Much of the following info can also be found at discrete synthesizers.
2600:
The very first model 2600 was produced in early 1971 and is known as The Blue Marvin. It has mistakenly been called the "Blue Meanie" but according to Alan R. Pearlman, the Blue Marvin is so named after Marvin Cohen, head of engineering at ARP at the time. It came in a bright blue aluminum case with a funky and fragile wooden enclosure. It used an entirely micro-chip based voice path but the chips they used wound up being highly unreliable, although they sounded great! Only a few of these were made (approx. 25) and they were mostly built in a garage.
2600C:
Next, the factory produced 2600C Gray Meanie (pictured above) appeared in 1971 and can be identified by the rectangular 'ARP' logo on its right speaker grille. About 35 of these were made.
2600P:
Later in 1971 came the 2600P V1.0 which featured the same metal case (housing the same electronics as the original models) now enclosed in a much more roadworthy suitcase-style enclosure (similar to the pic below). In 1972, ARP switched out the unreliable (Teledyne) VCO chips for the model 4027 and 4027-1 (National Semiconductor) VCO chips in the 2600P V2.0 and 2600P V3.0, respectively. These new VCO chips sounded weak and nowhere near as good as the originals. A new duo-phonic keyboard (3620) with LFO was introduced with the 2600P V4.0 in 1974. The 2600P V3.0 and V4.0 models are denoted by the "G-clef" ARP logo and are the most common models out there with roughly 1700 2600P's produced during 1971-1974. Unfortunately, further changes to inferior parts were just around the corner. The Model 4012 filter used in the all the 2600's through 1976 used a design that was a copy of Moog's patented ladder-filter design. This led to a law-suit between Moog and ARP which forced ARP to design their own filter for their next version.
2601:
The Model 4072 filter, a four-pole low-pass designed to replace the original 4012 was widely considered to be faulty. The frequency response of this filter only goes to about 12kHz, giving this model a duller sound than models with the Moog style 4012 filter. The early 2601 V1.0 models were still using the disputed 4012 filters during 1975-76. In 1977 ARP started using their new 4072 filter in the model 2601 V2.0 with the modern orange-on-black design (pictured below). Around 1978 ARP unleashed a whole new line-up of their synthesizers using this new orange-on-black look. These were produced from 1977 through 1980. Very late 1980 versions of this model featured un-potted submodules, making them easier to maintain (but they are more rare).
There were also three versions of the remote keyboard: The Model 3601, which is a very rudimentary version with just Portamento and Tuning controls. The Model 3604 is similar but adds adjustable scaling and was used until 1974. The duo-phonic Model 3620 (pictured at top and bottom) which adds a dedicated LFO, latching and some other very useful features was introduced with the 2600P V4.0, and used with all subsequent models from 1974 to 1980."
