I had to replace my crankcase cover (right side) on my 91 Suzuki DR 250S.
The part had been discontinued but the one for a 99 Suzuki DR350SE is supposed to fit.
I got the part and it has a big threaded hole right below the oil filter cover. It's funnel shaped at the very top, then threaded. It looks like an oil drain plug with a rubber ring would fit it, it's 14mm.
Its not on the old bike and I can't figure out what it is. A view window? Output to oil cooler? It leads directly to the crankcase and down that tube on the cover that I think is another drain.
I've seen pictures online where it just looks plugged.
Here's a picture of my parts, and another picture of some bike I saw online.
My service shop has no idea, and I called my local Suzuki parts dealer and they have no idea.