Two very early examples. The top piece is from 1918, and carries the #24 stamp (the size of the “Inner Tube,” not the shape). This pipe would later be referred to as the “O” in the catalogue. The bottom pipe is a 1925 PO shape in exquisite condition. From the G.L. Pease Collection 
A 1927 Dunhill Shell Patent Pipe 120 Shape Bottom: A 1965 Dunhill Shell 120 Shape - Note how the size, depth of sandblast, and overall design changed over time. It should be noted that the 1927 model has a replacement mouthpiece, and has had a shank repair. Early 120 shape pipes are notorious for having thin and elegant shanks that are prone to breakage - Courtesy of Mike Ahmadi
Introduced in 1996, the "Ring Grain" (RG) was created by blasting a straight grain bowl. Ring grain pipes in the Cumberland finish are designated "Shilling". It is an interesting variation on the original sandblasts which were mostly cross-grain sandblasts. The straighter grain plays a much more prominent role, giving the pipe a very distinct look and feel.
These are considered the best new Dunhills by many enthusiasts today, and are very rare. The finish is sometimes described as tasting like vanilla at first, with the taste becoming more normal or good as the pipe breaks in.