I think the standard you are looking for is B56. There are several versions depending on the type of truck. (B56.1 - industrial, B56.6 - rough terrain, etc.)You can download them at
I don't have a copy of the standard at home but I'm pretty sure attachments at a minimum have to be marked with the weight, max operating pressure (if hydraulically powered) and rated capacity. If the attachment has forks, they too must be marked with the capacity. For your own internal use they usually are also marked with your company name, a model number, and sometimes a serial number.
And far as putting new plates on a CAT(Nissan) forklift with your attachment, I believe that to be OSHA compliant CAT would have to supply the new plate, period. You have a load rating for your attachment, but CAT is responsible if the whole machine is unsafe. For example, only they know if the mast is designed to handle the loads you are generating. The OEM also has to consider not only forward tip, but also lateral and rearward tipping (especially with trucks that operate in the dirt). If you supply the plate YOUR neck is on the line. And unless you tip test each truck configuration your attachment is fitted to you will loose your a## in court when (not if) someone gets hurt/killed.
I also looked at Cascade's website. "For exact determination of the attachment/truck capacity, you should contact your truck manufacturer. Cascade Customer Service will supply you weight, lost load and center of gravity figures for the truck manufacturer's calculations."
As a side note, I was remotely involved with a lawsuit one of my former employers was defending. The operator was a 55 year old carpenter. He openly admitted to having a 1/5 of vodka and a 6 pack of beer for lunch. One front tire was not ballasted and it had a sticky throttle. He wasn't wearing his seat belt. He backed kitty-corner into a ditch with the telescopic boom in the air. When the machine started to tip he tried to jump, but the cab landed on him and he lost his legs at about knee level. He sued and won a significant figure because the insurance company decided it was cheaper than fighting him in court. --- I'm sure that others in the forklift industry have many similar stories.
ISZ