Traveller,
These caps were popular in the early part of the war (mostly 61'). The 1839 caps Came in many styles like Mexican War style (dark blue), civilian tarred mechanic's version, cloth types, etc. The Sicillian Caps were even more short lived; they were basically a bag like fez with a tassle on the end and worn on the head or over a kepi. I've included some of the pics I have, and hopefully they help. Both cap types would have been very common at Manassas.
Painting by Don Troiani - Jackson at Manassas.
Original Sicillian Cap.
Repro Cloth 1839 type cap.
Cloth 1839 type
Tarred Mechanic's Cap - 1839 style.
So you can see there's a lot of variety when painting these up!