The Number of Lines at the End of the Left Ribbon
This can be a little confusing. Any one can see there are clearly two lines at the end of the ribbon for types I, Ia and II, and three at the end of Type III. Technically the second line, and the second and third lines in the type III, are lines of shading. One of the easiest ways for quick identification is to look for a "V" or a "IV" at the end of the left ribbon. Only the type III has the "IV", formed by 3 lines, or 2 lines of shading, as it were.
If you still haven’t narrowed it down to a Type I, Ia or II Vs a Type III, take a look at the right ribbon. In particular, the extra line of shading in the Type III stamp is sometimes hard to see. It is always a good idea to check the right ribbon for verification.
The Number of Lines at the End of the Right Ribbon
It should be noted that the Type II stamps are usually much sharper and printed in a heavier ink than Types I and III. This shows up quite clearly in the right ribbon examples above. Note also, that the Type Ia stamp is usually a shade of rose carmine (slightly pinkish) clearly distinguishing it from the other Types.