I think the unshrouded, exposed ejector rod "D" framed guns of all types ended around 1972 with the introduction of the heavy barreled Detective Special and Cobra models.
From then on, all "D"'s had shrouds, including the Police Positive.
Up until the mid-90's the shroud was a short length.
In 1994/95 the shroud was changed to a full length version.
Keep in mind, for much of the time from the 1960's to the last production in the late 1990's, the Police Positive was usually uncatalogued, but was produced on order, often for foreign police agencies.
As always with Colt, they'd make guns to order with variations in barrels.
For domestic commercial sales, I DON'T think Colt produced any non-shrouded PP models after around 1972.
In all probability, the unshrouded PP was simply mis-dated and was not produced in 1979.