This is a rubber part with two metal balls that ensures the tonearm stays in the right position for either multi play or single play mode. The parts can be seen in the original Dual manual under the part numbers 213 and 214.
If this rubber is worn out, it will cause the tonearm to be in the wrong position. This is clearly noticeable, since the tonearm will encounter mechanical resistance on the underside of the player. Often, the needle will skip somewhere during record playing.
Luckily this problem can be easily solved by cleaning out the area underneath the multiplay sleeve and replacing the two metal balls which keep the tonearm in place to ensure smooth operation.
Please note: per product, 1 rubber ring and 2 metal balls are included (enough to repair one record player).