It is a CA-610, which seems to be the European or at least a non-US version.
From another thread here a while back:
"in the US, the only version of this amp sold (AFAIK) was, as I noted above, called the CA610II. Only recently (thanks to eBAY) did I learn that there actually WAS a CA610... looks identical to the CA610II, except the balance control is a separate little knob, rather than concentric to the (BIG) volume control."
I do really like the sound of the Yam. It's truthful, honest and doesn't varnish the sound overly. But damn those Marantz receivers are pretty!