Below is some info from the net. From that I would guess yours is between 1968 and 1972.
When the assets of Ithaca Gun Company were sold off in New York in 2005, we
did not receive the serial numbers for the Ithaca SKB Models. We do not
know who - if anyone - received those serial numbers, but here is the
information we do have:
Double Barrel Shotguns, Side-By-Side (SKB association): The SKB Models were
manufactured by the SKB Arms Company in Tokyo, Japan and imported into the
United States for Ithaca. Two new Ithaca-SKB grades were introduced in
1966; the Grade 100 and the Grade 200E (ejector) both grades have selective
single triggers, automatic ejectors, 25-inch to 30-inch barrels and were 12
gauge; by 1968 both were available in 20 gauge. The blued frame of the Grade
200E was changed to a silvered frame in 1972. The Ithaca-SKB Grade 150 was
introduced in 1972. This model was essentially the Grade 100 with a
beavertail forend and figured walnut stock. It was discontinued in 1975.
The Ithaca-SKB Grades 100, 200E and 280 were catalogued in the 1976
catalogue but none in the 1978. The Model 280 had a Walnut, straight-grip
stock. This was the last of the Ithaca side by sides.
Double Barrel Shotguns, Over/Under (SKB association): The SKB Models were
manufactured by the SKB Arms Company in Tokyo, Japan and imported into the
United States for Ithaca. The Ithaca-SKB Grades 500 and 600, initially in
12 gauge, were introduced in 1966. Both models had single selective triggers
and automatic ejectors. The Ithaca-SKB 700 was introduced in 1969. A Skeet
combination in Grades 600 and 700 was introduced in 1970. By this time, most
grades were also available in 20 gauge with 3 inch chambers. In 1973, the
Ithaca-SKB Grade 680 was introduced. The primary feature of this model was
an English-style butt stock. All of the Grade 700 models were discontinued
some time in 1975. The remainder of the line was discontinued by 1978.