Cancels On Great Britain 20's

Hi all! I just found these GB 20's in an envelope. I would like to know what is a good site or book to use as reference material to assist me with the numbers? I thank you in advance.

"Collect British Postmarks", published by Stanley Gibbons, lists these.

There are five types of numerical obliterator:

Number in diamond frame within an oval of horizontal lines - London, Inland Section. The number here represents an individual canceller rather than a particular office.

Number in circle within an oval of horizontal lines - London District Post and suburban offices.

Number in oval of horizontal lines, two vertical curved lines either side of number - English and Welsh offices. Here each number represents an individual office. Sometimes numbers were reallocated.

Number in rectangle of horizontal lines - Scottish offices. Here each number represents an individual office. Sometimes numbers were reallocated.

Number in diamond of horizontal lines - Irish offices. Here each number represents an individual office. Sometimes numbers were reallocated.

The numerical obliterators could be used on their own, or in conjunction with a circular datestamp to form a "duplex" cancellation. The English office ovals could be horizontal or vertical.

First image: 666 Ryde; 635 Reading; 713 Sittingbourne; 285 Exeter (this type is called a sideways duplex); 625 Portsmouth.

Second image: 50 Barnstaple; 53 Bath; 56 Forest Row; 408 Ivybridge; 466 Liverpool; 77 in circle East Dulwich; 447 Leeds; 498 Manchester; 132 Brighton (also looks like a sideways duplex).

Third image: 186 Dublin (Irish type); 720 Somerton (Somerset); 107 Bradford (Yorkshire); 5 Accrington.

Fourth image: 75 Birmingham; 306 Frome; 493 Maidstone; 279 Epping.

Hope this helps.

Phil

What Phil said

Numerals on Great Britain 1d reds are an enormous field.

John Parmenter's work is indispensable.

There is a wealth of information here:

http://www.gbps.org.uk/information/

Awesome! Thanks you both! I appreciate it.