I just posted this reply on the Other brands forum:
"I recently bought a 4.5" OMT and posted an inquiry on the Colt Forum. The response was that the 6" and 7.5" were standard, and the 4.5" was cataloged only for a very short period. 4" and 5" were available, but never cataloged."
If referring to the Colt Forum is prohibited, please delete. If allowed, can I add link to thread?