Hi Jim,
The tender deck plate is a (generally) slightly curved plate that is fixed to the back of the cab footplate, and is pivoted – this plate fills the gap between the cab and tender, and rests of top of the front plate of the tender. Early ones were almost flat: I wonder if the were sometimes caught on the tender, or perhaps the edges of the firemans boots did?
It will stop the fireman getting his boot caught in the gap which changes in size as the loco and tender move, for instance on curves but they can do it on straights as well!
Here in the UK they were in use from 1870 or so (from photos).
Looking carefully in the book on Baldwin Locomotives I think there is one on a wood cabbed consolidation from 1881. They are difficult to see with your narrow american style cabs (depressed centers, which were on top, or possibly below the top edges with some loco's, of the bar frames, with higher sides above the driving wheels ), as the rear support brackets get in the way of seeing them!
Cabs like this I think would have had a narrow version of them, possibly slightly wider the the depresses center: though I am not sure. Rear photos of the cabs are a bit 'thin on the ground'.