You have a 420SEL. But it is possible that one of the previous owners put a 5.5 litre V8 into it, which would (to some) justify a 560 badge.

to quote Snibble:

"Open the hood and see how many "snorkels" you have going to the airfilter. If you have one on each side(2 total) then its a 560. If you only have one snorkel to the airfilter.. then its a 420."

If they upgraded to the 5.5L, I would assume they would have both snorkels.

-gil