Last November, Roy asked about an ungumed sheet of nine different octagonal Machins, the ones that were used for postal and stamped to order (STO) stationery early in the Machin history. The post office allowed labels to have the STO Machin head, and some were produced as self-adhesives.
Douglas Myall notes "just prior to the withdrawal of the STO service on 31 October 1973 a small quantity of self-adhesive stamps were produced philatelically (but are valid for postage)." He lists various labels but not this full sheet of nine different values.
Recently I saw one of these sheets offered on eBay. It is being sold by a established dealer of GB stamps that has been on eBay for quite a while. He offers it as "Sample sheet - third printing self adhesive octagonal 1/2p to 5p set." In the description he notes "These stamps were printed on an experimental basis in very small quantity by Her Majesty's Stationery office. Very limited issue. Rosen Catalogue."
I'm not familiar with the Rosen Catalogue. If you know of it, please tell us in a comment.
The sheet is offered for £249, though it is also open for best offer.
At least this dealer believes it is a legitimate and apparently scarce item.
UPDATE: Douglas Myall sent me a note to remind me that he wrote about the octagonal Machins in his Deegam Report 104 last August. He doesn't discuss this sheet of nine, but he confirms that series three contains nine values issued between September 8, 1973 and January 8, 1974. Typically these were produced in sheets of 12 stamps of a single value. The series three self-adhesives were round with the octagonal design on them.
By the way, if you collect Machins and don't yet have The Deegam Complete Machin Handbook, you should definitely get one. Your purchase entitles you to free periodic update reports sent by email. Visit Douglas Myall's web site for more information.