Suzuki 2 Strokes

i have this master cyl cap going spare cheap .small graunch as shown in pic .my 550a has a bad graunch on its one and my mate with a pristine 750a had this as a spare but the thread fractionally too big .i do have a kettle m/cylinder for my twin disc set up so am confused as to why it wont fit .anyone who wants it let me know.Francos do them for £40 or so but sensible offer.

Not looking for much .just a fiver and post cover .

Or free if anyone needs it .wasted sitting on my shelf .