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Turns out the power head casting identifies it as a 135hp probably from 1970-72 the first of the direct charge engines.That looks like a Belgian serial number that was produced sometime "after" 1978 (my Belgian numbers are incomplete, but they were only into the 917XXX's when they started the 1978 production run).
The only listings I have for actual 1500 XS models were produced in 1974, 75 and 76 with serials between 4868648 through 5327662.
Of course it's always possible that the motor has been (cobbled) somewhat due to an issue - a Belgian bracket (or whatever) with that serial could have replaced an original but I wouldn't hold my breath trying to prove that....