Series I and II basses and guitars have never had plastic ovals as the default inlays. I suppose one could have custom ordered such a thing in the 1980s. I guess someone could have installed them in error.
The pearloid ovals first appeared on the Spoiler basses, as one of the ways to reduce the cost. The original material was ground up mother of pearl in epoxy and was pretty nice looking for the cost savings. Later, the supplier changed the material (without notice of course!) and it aged quickly, went totally clear and you could see the glue underneath.
Later we changed to all mother of pearl or abalone inlay materials since none of the models were made to be economy styles. The lower priced ones in our line now simply have no inlays as the default.
Glad you're enjoying your bass!