Don't believe the 400's and 426's had a grey wire.
Or you have an aftermarket CDI.
Or you're looking in the wrong place on that bike.
The 426 (US Ver) definitely has a grey wire from the factory.
Another tell-tale is the carb. The WR came shipped with the ACV circuit, the YZ didn't. The YZ has no coolant
reservoir under the left rear side-panel. The gear ratios are different in the trannys, although that would be a bit of a teardown just to check. Spring rates were different. The YZ didn't have any sort of lighting at all, nor did it have a kickstand. Another difference was the rear wheel.
The WR has a 18" rear, and the YZ a 19".
What kind of pipe does it have? A stock WR pipe has a spark arrestor, and an insert with a hole the size of your pinky finger. No clue what the YZ was shipped with, but it wasn't quiet.
Honestly, if it was titled as a WR, and you have a tail-light and headlight. It probably started out as a WR.