In the 70's, when Wal started making their basses as regular production item, it's where the "Pro" name came from.
It was offered with two pickups, or one pickup. And either Passive or active, and a few color choices. So were called Pro 1, Pro 2, Pro 1e, Pro 2e. And had pickguards and only solid ash bodies.
In the early 80's, they went a more fancier custom order route, and the "Custom 2P" was born. This is what you now see a Mk1 as being. Revamped active preamp and 2 pickups, mahogany core bodies, many wood choices and could be painted as well.
The Pro name was re-used to introduce the more basic model, the next generation Pro 1. This model only came with 1 pickup and is passive, no body contours on ash body, yes these still have a facing over the ash, seems most were maple. It's not as feature loaded as a Mk series bass, but you could add some custom options and paint color, which your Dad did add a nice facing wood.
These continuation Pro's are also known as 'reissue Pro'.