Now that this thread is a cement head, I'll post some listening results from years ago.
Kenwood offered a heavy ceramic platter mat, center weight and peripheral weight as accessories for the KD-600/650 turntable back in the day. We had the weights in the store and we put them on some tt, we usually had a Luxman PD-441 in the sound room. With just the weight there was some improvement but with both weights the bass tightened very nicely and the sound was more controlled. It was a very nice improvement but installing all that for each record was too much work.
Since then I have picked up a KD-600 that came with a record clamp that clamps onto the spindle, push down and tighten and a T-55c that came with the 486 gram weight. I use both when I feel like it. A difference, sure, but I'm not so serious about it to use all the time. Both tables were built to handle the extra weight.
You can see what the HK weight looks like with a web search for images. I like it because is has a strobe on the top. Good for my main tables which do not have speed adjustment, the quartz locked KD and a TD-160 Super.
I've asked on a different board and no one can identify the record clamp. Probably bought new in the mid-80s.