H# stands for Herz# and corresponds to the photo# in his chip book -- Nevada Gaming Checks and Chips.

P# stands for photo# (I think). These are his cross-reference numbers for the photo of the actual chip shown in THIS auction. The photos are actually in color, though published in this catalog in b&w. If you won one of these chips and later had it stolen, Howdy could look it up and provide you with a photo of the actual piece for i.d. or insurance purposes. (I assume he would charge for the service, and have never needed to use it, but that's how it works with his other auction catalogs.)