They are identified by a series of coloured numbers situated in the sheet margins for most stamps. It is important to note that in collecting cylinder blocks you must have them in the format laid down in the catalogues for them to be valid for collectors.
I specialise in collecting the ‘cbs’ for the pre decimal series of Queen Elizabeth 11 commemorative stamps and have several hundred to spare. Always unmounted mint and as laid down in the current Stanley Gibbons Specialised Stamp Catalogue - currently volume 3. It is worth the purchase price and can be picked up quite cheaply now that it is a couple of years old (in 2008).
They are becoming rarer because of the very fact that in most cases there is only one cylinder block in every sheet of stamps. There are always exceptions to every rule, for example the Concorde 4d will have 4 sets to every sheet and the Simon de Montfort’s Parliament 2/6d has no numbers at all. Like so many collectors, I have been guilty of breaking up blocks before I knew what they were, and so you can see why they are becoming rarer and, therefore, more valuable. The current catalogue values are already being exceeded on the open market.
Just like the individual stamps, there are variations, errors, flaws etc. which makes them equally more interesting, collectable and valuable. I already have examples of unlisted blocks in my collection, typical examples of which are the varieties missing phosphor bands or embossing. The catalogue does not make mention of these which are of particular interest to the serious collector.
A word of caution. Do not keep blocks of any kind in stock books. The reason for this is that the stamps can be damaged by compression marks. If the stock book is in any way squashed (other books on top for example) the little plastic holding strips can impress themselves on the stamps which are then de-valued or worthless to the collector. If you must keep them in a stock book make sure it is kept upright and not squashed between any othe books etc.
As with all obsolete stamps, there is a finite number in circulation. There is also a market in fakes, so you need to exercise a certain amount of care especially if spending a large amount of money. I have not found many Dealers carry stocks of cylinder blocks so it is not an easy market, but thank goodness for Ebay!
Collect and enjoy.
As a post script at July 2008: I have noticed that the older and rarer blocks are now far exceeding their catalogue prices and some of the lesser quality blocks are coming out of the woodwork. Make sure you do a condition check before buying any expensive items or you could be disappointed.