It is likely stamped on them somewhere. But yes 86-87-88 is when those where out and quite popular. The D suffix denotes the diagonal release feature of that heel piece. I'm not certain what year the 390D (4-10) was replaced by the 490D (4-10), 1986?. Then I think in 87-88ish the next 490RD (6-12) was released (pun intended), and then for good measure the 490RD metal (6-14) was on the market.
These were popular with powder skiers, especially the ones that liked to play in the woods. You could just give the heel latch a tug upward and you were out, even if upside down in a tree well!
Some pix...
Print ad from Oct. 1987
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390D and 490RD
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the 490RD 6-12 and 490D 4-10
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And the metal 490RD 6-14
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