I just put some rare mint alphabet bricks into my Bricklink store. These date to 1958-62 (they have the 1955-62 LEGO font on the studs), and were only produced in continental Europe and Britain. These are navy blue, and have a SERIF font. All later alphabet bricks (and all USA/Canada early/late ones) only had a San Serif font.
I don't remember where I got these... they came in a large lot long ago... but they are pure white Cellulose Acetate 1x1 white bricks, and will not lose their whiteness (CA has a tendency not to do that). These can be used with ABS LEGO bricks without noticing the difference (unless the ABS isn't pure white!).
These would have only been sold in continental European (and UK) #234
Alphabet Bricks parts packs. But since there were 50 to a pack... this is about 1 1/2 packs worth. I believe these were of German origin, since they have 2 (each) "ä", "ö" and "ü" bricks with umlauts.
These are only USA 45 cents each, and the total 76 bricks is $34.50. They can be found in my LEGO store and are the only mint white Alphabet Bricks in Bricklink.... (under decorated bricks)...http://www.bricklink.com/store.asp?p=istokg
See an image of the entire group below... (only 2 #234
sets on Bricklink... and both have used bricks). Also, each of these are of equal whiteness... just the way they're angled on the baseplate gives them a slightly different hue.