I think if you provide your VIN we can nail it down for you. I am not sure what number the model changed from 3-speed to 4-speed, but I think there were fewer 4-speeds made. But I also believe the 4-speed just has a starting gear to make it quicker off the line, but the 3-speed is a little more durable. That's my understanding.
I believe you will most likely have a 450slc 5.0 with that badging. Not may 500slc owners would see much point in adding a 5.0 badge (in my opinion) whereas 450slc 5.0 owners might have a minor inferiority complex.
You can probably check the EPC or the registry yourself if you don't want to post/provide your VIN.