For sale is an original USA-made Rickenbacker-branded 'No Mishap' strap, dating from circa 1950s/60s.
 
An interesting piece - I believe these were a Bobby Lee 'No Mishap' strap, branded specifically for Rickenbacker guitars distributed in the 1950s/60s.
 
This is the rather nice very dark brown (almost black) variant, with a tan back and woollen strap pad inner lining.
 
The Beatles have been documented using Bobby Lee straps - naturally being a Rickenbacker-branded variant, it's more than likely that they came across this variant with the many new Rickenbackers that they were supplied during the 60s.
 
In decent used condition, fully functional albeit with some fraying and tarnishing to the woollen pad lining.