So they might be considered a "semi prototype".
Both the 1530 all steel rerference called Oyster Perpetual Date and the ref 1630 Steel gold variant called Oyster Perpetual Datejust where introduced in 1975 and produced till year 1977 (a few were assembled in very early 1978) when the Quartz version was launched.
The 1530 was the first Rolex fitted with a sapphyre glass.
Best, Abel
PS : I’m sharing a picture of my 1630 which regrettably has a wrong bracelet which belongs to a TT Quartz model. The correct bracelet should have three gold links in the middle.
Probably the original owner had the original broken and couldn’t find a correct replacement. This example is one of the last watches made as per its serial number.