I know this is not original but I install parking brakes on all of our bikes due to the steep terrain we ride on. I first saw an improvised parking brake on a barn find MK4 auto bike about 15 years ago. I was able to identify it as an exhaust hanger o-ring from a Volvo P1800 2 seat sports car. The P1800 was built from 1961 to 1973 so at least the o-ring is period correct.
I store the o-ring on the left side since it falls off on the right side due to the throttle cable. The left side has the clutch lever. The brake lever is on the right.
I have been using these on our MK1s for years. The MK1s have the brake lever on the left. This really screws me up when I jump off the MK1 and onto the MK0.
I can store it on the left side with the brake lever on the MK1s.
I use a Yamahaha quad parking brake on our MK3 and MK4 bikes.
I retrofit the Yamahaha parking brake trinket to the original Magura brake lever on my MK8 bike.