I rarely every shoot M42 and I have two TCs, might as well let one go...
The brand is "Computron" never heard of it, but it seems very well built, multicoated and from just mounting it on the camera seems to be of good quality. I've never shot anything with it, though.
Anyways...I have both caps for it, and it's in 10/10 condition.
I have no idea what to ask...on ebay they seem to be going for about $15 US plus shipping for the third party ones...but, throw an offer, I won't be offended by lowballs
Also, I'm open to trading for 35mm film (esp C41 process B&W), Canon FD, Zenit and rangefinder stuff