Tim and Charlie... and everyone else who might have an interest:

Observations and thoughts...
In my original response(s) I did not consider the the other classes because the physical features of the car indicated Fe-19 to me.
1. The cars get bigger with time:

1. Fe-5 inside width 9-2, inside height 10-4
2. Fe-12 inside width 9-3, inside height 10-5
3. Fe-15 inside width 9-5, inside height 10-5
4. Fe-19 inside width 9-5, inside height 10-6

2. Some physical details change with time:

1. Fe-5: at the corners the end plates wrap around and are riveted to the side plates as is seen in early steel cars, the ends of the body bolsters (or bolster extensions?) appear as an I-beam, the side sill reinforcement is confined to little more than the width of the door opening
2. Fe-12: Photo needed!! 
3. Fe-15: at the corners the end and side plates are covered by an angle riveted to both, the ends of the body bolsters appear as an I-beam, the side sill reinforcement is confined to little more than the width of the door opening.
4. Fe-19: at the corners the end and side plates are covered by an angle riveted to both, the ends of the body bolsters are covered by a large channel "tab", the side sill reinforcement extends a side panel width beyond the outer edge of the main door (though I have seen one photo of an early member of the series with the short reinforcement).

The features of the car in question is identical to #4, a Fe-19 as is the car nee #5792 in the Arizona Ry Museum. 

The end features of the cars are apparently NOT class specific - at least that anyone has been able to determine. Rather mixes of "reverse" Dreadnaught and reverse Murphy panels and blank sheets are used in all classes (and in 50-6 cars being rebuilt at the same time). In both cases the two ends of the car may be different.

Some of the cars were renumbered: