On the subject of different coupler arrangements on front and rear, I looked closer at the 38 photos.
The photo below has the sliding coupler on the front. There are what appear to be reflectors on the side sill. The date is September 20, 1948.
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cdm16079.contentdm.oclc.org]
On the next photo there is a 3 way pocket on the front. You can see part of the "F" on the side sill behind the handrail. There are no reflectors? on the side sill. Photo date is 1946-50.
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cdm16079.contentdm.oclc.org]
Since I doubt the reflectors would have been removed, I believe this photo indicates a change from the 3-way pocket to a sliding coupler on the front.
Bruce