Greetings
Neilseitz, and welcome aboard,
To start with, your Wingmaster is a genuine 870. When Remington introduced the stripped down "Express" version, they could have avoided a lot of confusion had they not included "870" in its designation.
Your Wingmaster Model 870 20-ga LW (light weight) was introduced in 1972 and was available in standard and magnum models. They have scaled down receivers requiring some special parts.
When looking at the 870 Remington
factory barrels tables you'll see Wingmaster and Express models listed. The difference is the finish: Bright blue or matte black. IIRC, they are interchangeable, EXCEPT for your gun. It uses only 20 LW barrels. At this time, Remington seems to be offering only the slug barrel for the 20-ga LW, but the
Barrel Exchange has plenty of LW barrels listed.
Wingmaster 870 12s and standard 20 ga guns use the same stock. But, the 20 LW stock is smaller due to the smaller receiver. IIRC the difference between a LW and a LW Mag stock is the LW has a butt plate and the LW Mag has a recoil pad. I didn't see any LW stocks listed at the Remington site, but
Numrich Arms has a few.