Because, with all due respect to our favorite author, base-16 has 16 digits, including 0. And the digits used in book are 1, 2,... 16 and 0 (in the map). Yes, I'm a bit of maths geek.I guess he meant the number system used is based on number 16 (which it is), but not in a strict mathemathical sese of base-16 number system. Kinda like Roman numbers are based on 2 and 5. Or maybe we shouldn't assume that the numbers used in the book are the exact in-world number system. (=I guess mister Sanderson or his editor may have done a little mistake here—they're writers, not mathematicians. Therefore overanalyzing the exact way numbers are written in the book may not be a very good idea. Let us enjoy the fact it's based on number 16 and overanalyze the Ars Arcanum instead )
And for edition - I think it's either a word written in steel alphabet, or (more likely) a date or something similar. Not one number - if it was on base-16 number, with that uch digits, it would be to big to resonably fit as a number of edition od a newspaper. Also, it has two symbols that aren't in the 0, 1, 2,..16 set. Some kind of separators, maybe? Like "/" in "edition 12/11/2011"? (sorry for European format)
I think the other 2 symbols (19 and 20) are not metals, just additional letters. They may be, however, some atium or lerasium alloys.