I think you probably have a cheapo knockoff... way too shiny. I'd go with a new Yamaha one... cheap or expensive Yamaha part doesn't matter... with the ebay ones you don't know what you're getting. The Yamaha ones are good quality.

My 76 Honda CB750, petcock was on the right side with off being lever forward. My 73 CB500 was on the left, off pointed back. Its a crap shoot on those bikes. For quit awhile I was parking my CB500 with the petcock in the reserve position, fine until the needle valve started leaking.
Now that a thought about it, I think it had a early tank, eventually all bikes had petcock on the left.

1969-81 Yamaha fuel gas petrol petcock gasket pet cock dt250 dt360 xt500 ct1 at1

$29.97
This is the Yamaha part, has the manufacturer stamp (TK)
The four I purchased before this one came from rocky mountain and partzilla. They were $70 and are Yamaha parts.
The E-Bay one is Identical, same exact stamping.

Not sure how to put the e-bay link in. if you copy and paste description in search box it does come up. seller is = retro-motorcycles
fast and free shipping

This from ebay has been on my DT360 for a few years now, no leaks and cheap.

MarkT on the description on that petcock it says “aftermarket” a lot of sellers are getting creative trying to sell what amounts to a chinese knock off, in this instance they put something in the picture with what looks like a box from Yamaha to try and pass them off as genuine Yamaha parts. Although some users have had good results using the $7-$9 dollar “aftermarket” chinese petcocks?

Mark was helping me with the e-bay link.
The part comes in the box I show in my picture ( Tour Max) made in Japan on the box.

The T K trademark cast into the part is what is on the petcocks from Yamaha .
Yamaha does not make all their own parts, I.E. Mikuni carburetor parts are made by Mikuni sold in a Yamaha bag with a Yamaha part#
Typically the trade marks will be slightly different to avoid copyright infringement when being counterfeited.
I can see no variance in this instance.

All I was doing is posting the link for yamahoper... he says it's a quality part exactly like the one Yamaha sells... I know him personally... he's a stand-up guy and I trust his judgement that he knows quality parts when he sees them...

TK = Teikei

I've come across their carbs here in the UK on the RD125 and RD200, standard Yamaha fitment on those.