I have questioned for years the meaning of and "X" on pre and postwar boxes, cars and trains. My 90% conclusion is that it simply means "different" from regular production or last years production, or special order uncataloged item. The 2812X is the only late 2800 freight with a low-hood coupler, X on the box only.
2452X PRR gondola is on the car bottom and on the box, truck or brakewheel or steps variation???
1666 locos can have an X inside the cab in different locations, underside of roof or on fire doors, also a variation with the X part of the cab number. I think also on the boxes, but what is the significant change that warrants the X
??
Orange stripe 2338 Mil Rd diesels have an X on the box, this makes sense except wasn't the orange stripe first production, making the later black cab the variation, so it should have the X. I have seen both variations in X boxes.
The tuscan B&O 6468 automobile boxcar has an X on the box, regular production is blue, not sure which was first.
I had a 2353 ATSF F3 diesel box with 2 (XX) after the number on the box.
The 156X early electric has no pony trucks, never had a box for one, not sure if the X is on the box. The 1922 catalog first shows it in set # 169, but no X designation. 1923 does show the maroon 156X, does list it separately and in set 169 with the X designation, so "different" is the reason for this X. The 156X is shown again separate and in set 169 in 1924, but in 1925 appears to have been replaced with the 253.
This rambling list is just the tip of the "X" iceberg, there are many more, but my vote, 90% of the time just means "different".