The game is copyrighted 1981 on the introductory screen; a reference to an earlier version? See also Mutant Invasion by the same author, which has a 1982 copyright date. Perhaps the game was developed originally for TRS-80; like the author's Adventure International-published game.
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Both this and Mutant Invasion seem to have their origins in earlier games, given the 1981 and 1982 copyrights. Perhaps TRS-80 titles? Michael's Merlin's Treasure was published for the TRS-80 by Adventure International.
Inconclusive. My copy is marked as version 1.2, but its dating is doubtful. The executable looks assembled with an early version of MASM, which did exist since October 1981. It is possible (even if unlikely) that this is a PC original, coded from scratch. Unless anyone comes across evidence of prior art, we can only guess.
I don't for one second think that the PC version itself dates from 1981. But the fact that it has a 1981 copyright date, and another of his games has a 1982/87 copyright date, and he also had a commercial release in 82/83 suggests that he probably produced a TRS-80 version of Intercept in '81.
I'm guessing it's a 1987 date for the PC version, just like Mutant Invasion. They're obviously programmed in the same way and they begin to crop up in shareware catalogues regularly from about '88 onwards.