This article is from the RCA SelectaVision VideoDisc FAQ, by firstname.lastname@example.org (Tom Howe) with numerous contributions by others.
With most program material less than one hour in length, and with most
movies requiring two discs, there is an unused side with "nothing"
recorded on it. But on inspecting these unused sides (usually stamped
with the code 99999) there is still a band of grooves about 1/2" wide.
Some collectors have speculated that there may be something viewable
hidden away in these grooves, but on the few discs I checked, I found
nothing except black video with the last 6 bits of the DAXI code
translating to Band 63. This band number signals the system control
microcomputer to initiate the end of disc sequence, causing the player
to immediately raise the stylus off the disc. Still, it is possible that
the Band 63 code is only recorded for the first few grooves, but on the
discs I checked, this was not the case. On the J and K series players
there is an easy way to spot check the complete width of the grooves by
loading a disc and turning the power switch off as soon as "E" is shown
on the LED display. With the player's cover removed the second reduction
gear can be rotated slightly to the rear of the player, causing the
pickup arm to advance over the grooves (a label on the inside of the
cover illustrates the location of the second reduction gear). When the
power switch is turned back on, the player will display whatever video
signal it finds directly under the stylus, or the LED display will show
"E" if the Band 63 code is still present (never rotate the second
reduction gear with the power switch on, as this can break the gear or
damage the stepper motor it is attached to).