1978 Kawasaki KZ650!

I think your muffler are from Jardine Performance Products. The company was out of California and pretty popular in the 70's and 80's. They even had some collaboration with Kawasaki apparently.
The pipes were typically straight, wide with a turnout tip.

I have these pipe on my LTD in chrome. I've never seen them in black, but perhaps yours were painted later.

They are fine, put I'd like to replace mine with an OEM version as the Jardine as a bit too loud and smelly for my taste.