Welcome aboard....
Ithaca imported the 600, made by SKB in Japan. IIRC, they started in the late 60s. Pop had his by 1971. A fine little 20 gauge that kicked it in the uplands.
I recently acquired a similar 600 in 20 gauge. It's a claycrunching death ray and does really well on Dove. In VG condition, it was OTD last year for less than $500.
HTH....