hey chris,
after looking at a model serial number decoding chart i have from promatch that covers older lifts up to around 2012. I'm wondering about this model number and serial number you've posted. I do know clark was pretty crappy when it came to stamping their dataplates and sometimes their numbers didn't come out clear.
I wonder if possible the model could have been a C500-40
and the serial number may have been something like 355 817 4631 11-81
I'm not 100% sure on the serial number part though, i do remember when i ever went to a clark dealership they did want the serial number because they had some special decoding system they used to get the correct literature on the truck. The end of that serial number you gave 'might be' a year but i doubt it because the decoding section in my promatch stuff doesn't mention any years. I think hyster and yale were the only ones that did that designation in their serial numbering system.
You're correct though to go to clark and have them help you with this.