Sorry for the late reply, I missed the notification.
Indeed, on the PM6681 the 20 cm cable is not long enough. I will give up entirely on the 20 cm cable option as the difference in response between the two is negligible.
I also have some new 30 cm cables on order, which will be of higher quality, so once those are here, the choice will be between low cost 30 cm and high quality 30 cm cables. The main difference will be in the build quality and materials.
Regarding the Pendulum counters, yes, it should work there as well. It even works in the CNT-90 - had a customer who installed it in one but I think the connector didn't fit quite right and he had to shave two corners of the connector with a file.
As I'm writing this, I'm out of all types of cables, but I'll have some of the high quality ones in stock as of next week.
Edit: I have completely given up on the low cost cables.