Not sure how far back this goes, but when using 3.4.7421 P and 3.4.7414 P with this virtual tag
$If(<Disc Count>>1,$IsNull(<Disc#>,Null,"Disc "<Disc#>),)
the results are not what I get with any 3.3 version. If there is only one disc, it should not show a disc number nor anything else. With 3.4 if there is only one disc, it shows "unknown disc #" where it should show nothing.
With 3.4 and more than one disc:
(http://i.imgur.com/o5bommSl.jpg) (https://i.imgur.com/o5bommS.jpg)
With 3.4 and only one disc:
(http://i.imgur.com/58lLTvnl.jpg) (https://i.imgur.com/58lLTvn.jpg)
With 3.3 and only one disc:
(http://i.imgur.com/2DqeyXJl.jpg) (https://i.imgur.com/2DqeyXJ.jpg)