A post from a reader in response to the missing Nikon HB-44 hood:
"Here's another missing number (at least in the U.S):
The 16-85mm, released February 08 was model #2178 (link)
The 18-105mm, released August 08 was model #2179 (link)
The 50mm 1.4G, released September 08 was model #2180 (link)
The 35mm AF-S is model #2183 (link)
Where is model #2181 and #2182?
It could have been assigned to another accessory and not a lens, but Nikon usually keeps these numbers pretty much in sequence (flashes are in the 4000 series, for example). So has #2181 and #2182 already been assigned to lenses we don't know about yet?"