I'm just about certain the .410, 28 & 20 Express models are all on the same basic frame size. I would expect the Wingmaster is too.
The loading/ejection port sizing is different, however, the back end of the Express action seems to take any of the 3 gauge stocks. Can't determine any difference in how they are inletted, or for that matter, any diference whatsoever in the 3 different gauges of 870 Express Stock.
Best guess, a 20 ga, Wingmaster stock should bolt right on to your 28 ga.