These images show the three typical Source Four gobo holders for A and B size gobos and in which slot they can be put in. There are other holders available, however A and B sizes always remain the same diameter. Note that the "Drop-in iris slot" can only hold either a drop-in iris OR the wider gobo holder, not both at the same time.
( Tape measure in centimeters )
This applies to all Source Four profile models, including the XDLT shutter barrells for LED Series 3 as well as the ColorSource Spot (V) fixtures.