The appearance of a new version of the second class coil stamp from Walsall prompted me to review the issues since the introduction of the first one with the recent security features. Although this has been covered elsewhere, I thought it a good idea to review here, since there have been a number of unusual events.
The first self-adhesive second-class coil with the new security features appeared on 16 July 2009. It had source code MRIL ('R' for roll) but no year code, since year codes were not introduced until 2010. This coil was printed by De La Rue and has a yellow backing paper. A five-digit number is overprinted on the back of every tenth stamp, indicating the number of stamps left in the roll of 10,000. The De La Rue coil is pictured above.
In early 2012, collectors were surprised to find that a new issue of this coil, with year code 2010, was being used on mail by large mailers such as The Sunday Times Wine Club. Used copies rapidly appeared on eBay (and presumably other auction sites). The first copies went for as much as £100, but the price has fallen as more copies were discovered, and the going price is now around £5. Some unfranked copies are known; presumably these went through the mail but were not canceled. To this day, no mint copies have been found, and I believe Royal Mail has never officially acknowledged them.
Later in 2012, a new version of the coil printed by Enschedé appeared. It has the codes MRIL/MA12 and is on white paper. Like the first De La Rue coil, there was a number on the back of every tenth stamp.
Another surprise occurred in early 2014 when a second-class coil appeared with the codes MRIL/M12L. This one was printed by Walsall, also on white paper, but with no back-stamped numbers. This stamp is shown below, but the codes don't come out well when scanned, so you probably can't see them.
The second surprise of the year occurred on July 31 when Royal Mail made available a new printing of the Walsall MRIL/M12L coil. The main difference is that this coil has the numbers on the back, every tenth stamp as usual.
The easiest way to confirm that a coil belongs to the original Walsall printing is to have a strip of 10 with no number. These may not be easy to acquire. I asked one dealer if he had a strip of 10 available, and he said that all of his stamps were cut down to strips of five to fit in his stockbook.
The stamps from the two Walsall printings may be subtly different. One dealer reports that the second printing is a slightly darker shade than the first. I find this hard to see on my copies, so if there is a difference, it is very slight. Ian Billings points out that the iridescent ink appears to be yellower on the second printing. I can see this, but it definitely takes having both stamps together to tell the difference.
Here is the number on the back of the second Walsall printing. The number is reversed because we are looking at the back; the number would be correct for someone looking at the front of the strip after having removed the stamp.
That's the story so far. If you have any further information, please add it to the comments.