Tiny steel ball used for sliding the tapehead bridge when engaging/disengaging the tapeheads in the following Bang & Olufsen models:
Beocord 8004 HX-pro
A total of three leaf springs with each their tiny steel ball holds down the tapehead bridge while engaging/disengaging and helps to stabilize the tape head bridge during play and record operations.
The tiny steel ball is often lost when a leaf spring breaks off its holder inside the drive mechanism or when servicing and cleaning the drive.
Most common is to see the leftmost leaf spring (near the erase head) break off, but it can happen to the other two as well.
This is a direct replacement for original part# 2917018