Seriously nicely done. And what a great idea!
My only observation would be that I doubt they had spray bombs back in the CW era so your overspray isn't really period correct.
There's a couple of ways I can imagine them marking the boxes back then but I'm not sure which would have been used. The first would be a metal stencil and a semi dry brush of paint would be used to stipple the letters onto the box. The paint would have a slightly rough look to the coverage with perhaps the odd splotch or dribble where the brush would be too wet. Similarly some of the areas and edges of the letters would have a jagged look where the paint didn't take or didn't cover well to them because this was being done fast in a production manner and all that is needed is enough paint to be able to clearly read the letters. So it would pay to be a little quick and cavalier on this aspect.
The other method could be that a slightly curved "stamp" would be used to print the lettering with a bit of a rolling action similar to the curved blotters you see in some of the old B&W movies. The paint would be applied to the raised letters and the stamp rolled over the surface. Again, due to the wood grain and issues with the paint not fully covering the stamp the letters would be rough looking and have somewhat jagged edges. But neither method would include an overspray pattern.
If you want to have another go at it I'd suggest first sand off the present lettering. And then, since these boxes were made very roughly, I'd try wetting the wood to raise the grain. After it's dried use your stencil again but this time stipple the paint on using the tip of a stiff brush lightly wetted with paint and jabbed end on at the stencil letters. Be quick and don't try to make it look TOO nice. The kid or old grey hair too young or too old to join up doing the originals back then would not be too worried about making it look perfect.
But before you add the lettering a coat or two of boiled linseed would provide some subtle but appreciated protection for the wood. You'd want to do this after the water used to raise the grain is dried away but before the stencilling.
