I have ben collecting Great Britain Machin definitive stamps and have created a basic spreadsheet for keeping track/recording the stamps I have and subsequently the stamps I require to complete the set. currently this is intended to be a basic one stamp of each denomination/colour variant not venturing into the various nuances of each stamp ie type, source etc.
However while collating a comprehensive list of machins using the latest SG concise catalogue I could not find any stamps allocated with the following SG numbers:
X923 and X995
would be interested to find out if anyone knows why there are no entries or could advise the stamps with those numbers.
Please read the comment about X-numbers that is likely to appear at the start of the listing. These X-numbers to avoid having to renumber all catalogue listings when new values appear. SG lists by set and not in order of issue date.
In the 1989 Concise X923 was the 8 1/2p yellowish green issued 24-3-1976. X995 did not exist at the time. By 1992, they re-arranged the listingThey left a few gaps for possible new denominations to be issued and X923 became one such gap. X995 appears never to have been used. X923 was never re-assigned.
I do not have your edition, but my guess is X995 would be at the end of a sub-listing.
Collectors demand rapid assigning of catalog numbers (You see threads here about "What is the # for the stamp issued last month?) Conversely, postal administrations issue definitive series over a span of many years. The clash of the two will result in unassigned catalog number gaps or substantial renumberings. Take your pick.
In SG's case, they tried to group issues mostly by common attributes (printer or print method) and in denomination order. Given the long span, this resulted in many issues with some renumbering to try to maintain their idea of order.
They were initially in the main section (just like Scott) so their move to a separate Machin section left unassigned there. Yet, there still some Machins not following the X,Y, U scheme such as 1668 to name a number.
The Gold Machin was intended to celebrate the golden wedding anniversary of the Queen and Prince Philip. It was intended to remain in use for a year and then be withdrawn. As such, it was not intended as a definitive stamp as were the Machins. It was brought back for the Golden Jubilee of the accession in 2002 and has become a definitive.