The ARP company was founded by Alan R. Pearlman whose initials in case you didn't notice are "arp". The company competed with Moog as the primary player in the modular synth market before their more portable products the Minimoog and ARP's Odyssey hit the scene in '71 and '72 respectively. In 1981 ARP ceased to exist as a company and here is a great page explaining some of what happened. In any case they made certainly some of the greatest electronic musical instruments of all time. I strongly advise the reading of that article.
Here is a page that has pdf's of a lot of the sub-modules used in ARP equipment. A life saver when you find a dead potted module as in my own pro-soloist.
The engineering team went to work for CBS as noted to continue the Chroma project and of course they marketed it under the Rhodes name to associate it with their acquired keyboard product. The Fender Chroma Polaris came a few years later and it became the last of the group effort associated with the ARP company.
2500 2600 Pro-Soloist Odyssey Solina(s) Explorer Omni Omni2 Axxe Quadra Quartet Solus
After the early years ARP began labelling parts on diagrams with their own numbers. Here are some that I've come across so far, so that none need be confused:
SL19986=A2801-009=1400801=56019=LM301 Op amp
SL19988=A2801-008=1401101=56018=LM1458 dual op amp
And here's a note from Rob at Emulatorarchive.com
The ARP 2600 Service Manual explains how to replace a 1339 with a LM301.
You basically swap the 2n2F capacitor with 30pF. Swap the 390 ohm resistor
with a wire link, and remove the 10K resistor.
This will improve the sound too....
However you could easily use a better Op Amp and get rid of the 30pF
capacitor, such as a NE5534 or TL071.
Omni-2 Pro Soloist Prosoloist pro/dgx