Thanks Steve. I ended up putting it in front of my LB White propane garage heater for a few moments...industrial hair dryer! I could almost push them through with my gloved hand. Worked like a charm.
A theory: original unitized wire sets are not numbered on the top side rubber. After pulling all my sets out, I noticed my early dated set did not have the numbering on the top, dated 2Q 73. All my later sets, 1 and 4Q 75, 4Q 77, were numbered on top. Also noticed the older dated one does not have the little nub on #7 and #4 wire.
Can anyone else chime in on this? Here's a pic:
