No, they didn't. You're repeating a myth. Oft-repeated by others before you, but still not true.
There never was lawsuit and ARP did *not* use epoxy in their sub modules to hide anything. I've talked about this numerous times but I will keep doing so because as you say there's still people out there who believe things which aren't true.
ARP's designers -actually Tonus at the time- came from the US defense industry and the US space race of the 60's. Where it was common to embed circuits into submodules for thermal stabilization, resistance to thermal and mechanical shock and to provide a known sub-circuit 'block' which could be easily replaced when necessary. Having been tested and qualified individually. One company they worked with back then for their 1005 Modamp ring modulator potted sub-module still exists and is still making potted circuits and modules this way for industry and military use. Hybrid circuits in Massachusetts.
You still see this in the Audio Discrete Op Amp DOA modules plugged into API lunchbox modules. Where it is done for the same reasons listed above.
The idea that ARP-Tonus was hiding their use of a ladder filter is hard to maintain when you look at the facts. First, that moog was also using ARPs patented dual-voice keyboard at the time. <--That's why there was no lawsuit.
But more immediately obvious is that the ARP2500's 1006 Filtamp module, seen here in a view that requires one screwdriver and two screws removed to see.. Is quite clearly the moog ladder. You don't have to be an expert or need any great experience to see the four red capacitor rungs and quad red-topped matched pair transistor sides of the ladder on this PCB! The detail photo shows the same ladder arrangement but with tan capacitors 'rungs' and only one side of the transistor pairs painted red.
And here's the kicker. This module was in production and out in the world a year before the 2600 arrived with its potted sub-modules including the 4012 moog ladder filter. If they truly wanted to hide their use of the ladder, wouldn't this ladder be hidden??
PLEASE DO share this widely and with the authority of truth! It's time to put an end to these easily proven as false old narratives. And we have the power of the internet to make it better. Or to make it worse. Please help spread the word. The designers deserve not to be labelled unfairly.
The small 1" sq. potted sub-module seen here is the exponential CV generator for the filter. It has two temperature sensitive resistors inside and several transistors whihc would also be affected by temperature changes. It is potted in epoxy to keep all of these separate components at the same temp and to slow any ambient temp changes impact on this frequency critical portion of the circuit. It is not part of the ladder of the filter. The mechanical stability gained comes along for the ride since the temp concerns are the primary reason to pot this part. But the Filtamp was also available in a linear type. Which meant they only had to swap out this expo module for the linear type to change and fulfill that filter order. Mfrs didn't use potted modules to hide! They had good reasons to do so which had nothing to do with hiding anything. Like quickly changing a filter type from linear to exponential to match an order.