Speedmaster is Eseco, and they're still in business out in Oklahoma. We have another model in our lab, but the phone number on the manual is: (918) 225-1266. They have good customer service too, you might be able to get the manual from them. I'm not familiar with your Durst processor, but I do have an old 80s era Durst processor at home, that says Durst on the shell, but was actually made by Thermophot. They were imported into the US by various companies including Nutek, Durst, Jobo and finally Wing Lynch--who's the current distributor. I can tell you that nobody supports it anymore. There're no parts, service, tech support--nothing, zilch. If your processor is young enough,t Jobo has a timeline on their website of the support for Durst products, since they're the US distributor of them now. The Printos are about the only current paper processors Dursts makes as far as I know.
The number 502 though, almost sounds close to a Thermophot--they make an ACP 502 something or other processor now....here's a link to their website, if you're interested, otherwise, I'd try Jobo and Eseco. btw--jobo keeps copies of their old manuals and will send them to you for a nominal fee. I got one for a multitronic timer for about $5.
fwiw, I've used several paper processors, and currently use about two daily. If you need some help, I could try to answer a question, but I'm by no means an expert on them. hope this helps,