According to that latest english article, he says he bought his modulars when nobody cared about them anymore. He could be talking about the 80's here, as we didn't see him use the Moog 55 and ARP 2500 before the mid-80's. There is a a rare video though of JMJ playing an ARP2500 and a few other old analogues. That clip looks to be from his early days, which means he had the 2500 since long ago or at least borrowed one.
We know for sure that he used the 2600 on the Oxygene album, and that must the ARP mentioned in the sleeve.
Mr. Tung wrote:
Also, best to remember that the latest release of Oxy does include instruments that were not even made during the albums first outing.
The list for JMJ is the same as the original, but Francis and a setup is added including Jupiter-8, which was not made when the original album came out. Francis didn't feature on the original album either.