Dip-Less pens are so called because when used with the corresponding base which holds one to two ounces of ink (depending on model), one could "dip" less than when using an ordinary dip pen.  This of course is accomplished through the use of the pen having a feed.  While dip-less pens do have an empty cavity in the barrel, do not fall victim to the belief these are eyedropper pens.  This is ridiculous as the ink would never stay in the pen and is totally unnecessary when used with the base.

Here's a crude scan to show the differences between the X, XX, and W desk pens.  The top pen is your standard XT dip-less pen, the middle pen is the XX (correctly model XXT) size, note the larger and longer barrel, and the last pen is your standard WT model desk pen.  Note the taper is the same length as that on the XX and is interchangeable.  While not a huge difference over the standard size, the XX is much more comfortable for those who demand a larger pen.

This is an earlier dip-less pen that is bandless, similar to pocket pens of the period.  This is the correct style pen for early 401 series "drum" style dip-less wells, note the differently styled section.

This is a slightly earlier model than above as it is pre-"bandless" design, and still has the uniquely styled section.  As you can see from the second picture, the nib is held in place to the feed by means of a lever which releases the the nib and feed and can be removed from the section to replace the nib.  This style was designed to use the 5xxx series nibs.  Early models with red tapers were solid not translucent like later red tapers.

This is your usual model XT dipless pen, but this is made in England and has a gold plated 3312 Italic nib.