That is not a mono plug. A 1/4" "mono" plug only has 2 contact points on the end of the 1/4". That picture clearly shows 3 contact points. Usually the tip on a 1/4" is the "+" , Ring is the "-" and sleeve is the "shield / ground / drain connection" They are called different names but do the same function.
These plugs are also refered to as "Stereo" 1/4" plugs. Most people use pin 2 of a xlr as the "+", pin 3 as the "-" and pin 1 as the "shield" connection. There has been confusion over the years as to what pin is the "+" and what to use for the "-". Most have settled on pin 2 as "+" on a xlr plug.
The statement was made that the picture shows it as "mono" That is not a correct statement. It's considered balanced, just using 2 different kinds of connectors to get the same result.
The easy way to remember it is if the 1/4" has 2 black rings on the end of it is considered mono or unbalanced, and if it has 3 rings on it then they consider it a balanced or stereo connector.
I hope this may clear up the confusion.
If you have any more questions let me know and I will do my best to answer them. I used to work for a sound company and my thing was buliding equipment racks and custom length cables with different king of connectors on each end, so I learned real fast the difference.
System specs: HP Pavilion zv6130us
AMD Athlon 64 2.0ghz, 768 Meg Ram, 128 Meg video ram dedicated, 80 Gig H.D., Win XP Home SP2, 160 Gig external H.D., M-Audio Fast Track Pro.
PCDJ FX & PCDJ Red 5.2
System 2; Gateway m460, Centrino 2.13Ghz, 1gb Mem, 250gb External H.D.