The cause of poor loading performance from the Datassette units is often a perishing drive belt becoming slack, or more commonly kinked where it has sat without moving around the drive pulley for 30 years or so. This kink then becomes a permanent feature of the belt causing it to jump and slip.
The tape counter belts don't usually snap or degrade, but can slacken off over time and may start to slip.
These square section belts are of identical design and size to the originals but made from modern, more resilient materials. They are absolutely genuine spares.