First,, the base pad is the most obvious. Totally different. But the main thing,, is the notch in the MKIII mags to allow the mag release catch to fit in near the top. It also has a little "bump" at the notch as well.
As a side note, I have used a MKIII mag in a Ruger Standard model with no feed/misfire issues.
Ruger Standard mags have a "thumb assist" knob on the mag to assist in loading.
As I understand, the mags should have no interchangeability issues, unless you are trying the 22/45 variant; those won't swap with Standard/MK variants.