Ok - I'll do some rambling thinking out loud here
I have the E-74 sill figured out. It had the stamped sheet metal tire carrier on either the left or right side with the 4 bolt horizontal latch. The sill was a simple channel shape. It was covered with a skin that welded to the quarters and gate sides.
From 01/75 to 08/76 the sill had the recessed catch and lip for the doors but kept the older style carrier (stamped sheet metal) and latch. It was still a two piece affair with a channel shape and a skin. Left side tire carrier only???
From 09/76 to 12/78 the only change was the style of carrier from stamped to tube and the latch/ bumper to match. Left side carrier only???
The 01/79 to 84 sill had the tubular carrier and it was only on the left side (nothing is absolute though) - it had the rubber bumper and the angled smaller latch. The sill was composed of two stamped sheet metal parts that formed a rectangular tube. It had access holes on the top to get at the mounts, recess for the door catch, recessed lip for the doors, drain channels underneath.
Anyone have better info?
Kevin