JT Sprockets

I see the 630 pitch, JTF 518-15 and the JTF521-15 sprockets listed frequently as one and the same and for the same bikes... a picture of them both shows them not the same.
The JTF521 looks thicker (10mm) with a boss at the spline shaft.
Which one is actually correct!!??
I have a small box of sprockets in good condition and I have both darn it!!

Oops sorry... I'm thinking this through for my 77' KZ1000A,1 which I thought uses the thicker JTF521-15 but I've had older Z1s, KZ750s and KZ900s etc with their sprockets and all mixed up in one sprocket junk box.....

JT has a website. Plug in your model and out spits the recommended sprocket. I did just that and it states JTF521

Nessism wrote: JT has a website. Plug in your model and out spits the recommended sprocket. I did just that and it states JTF521

Yes.... just did that too... so I must have a couple sprockets that are not for this year or even this engine!