The "AO1" Kingston "Killer" Obliterators
There are seven types of this "killer" obliterator known, designated by letters "A" through "G"*.
Type A. The earliest form (Type A) was sent out from London on April 14th 1858 and may be found used on early GB stamps and on the "pine" watermarked Jamaican stamps. This die was in use until 1863. (Three further copies of the obliterator were sent out on April 28th 1858).
Type B. This duplex die was sent out (in duplicate) on April 30th 1859 with index letters in the date stamp of either "IB" or "2B". It was in use for some twelve years until about 1871.
Type C. This die was a double "AO1" form and was unpopular and little used since it required a second date stamp.
Type D. This single die with thick bars (unlike types A-C) and small heavy lettering was in use for some ten years from 1860.
Type E. This die has thick bars but with taller, thin lettering than type D, and an oval "O". It was in use from 1860 to 1875.
Type F. In use from 1870 to 1885 it has a large round "O", thick horizontal bars but thinner side arcs.
Type G. This die is intermediate in form between types E and F, with thick bars and roundish "O". It was in use from 1870 to 1885 when the use of the AO1 cancellations was superceded (January 1885) by the squared circle dies.
* Sources for this material:
Collett et al., (1928) pp. 198-203
Nicholson LCC (1949)
Potter IA (1996) pp.16-20.