Starting in 1916, plates were made of steel, and approx. 15 x 8 inches in size.
After reaching 99999, an A prefix was used in 1916, and then, for a very short time, an O prefix was apparently used as well.

Apparently, the painting process was not very precise on this first embossed issue, so some of the more poorly painted plates were touched up before distribution.
Because of this, some 1916 issues will have "natural" brush strokes on the numbers, applied during the manufacturing process.

For 1917, A, B, O, X, and N prefixes were used.
I'm not sure of the order in which these were issued, except that the all-numeric plates came first.

After going to 6 digits in 1918, the alpha prefix was no longer necessary and all-numeric plates were issued until 1925.

The plate below was stamped with the year 1920, and then overstamped for 1921.
This is the only overstamped plate known.
Leib Brothers

There are two distinct "5" dies on the 1924 plates.
One has a higher middle horizontal line, and one has a lower line.

"High 5"

"Low 5"

Both pictures from eBay
Looks like 1924 was the only year for this variation.