The Royal Gazette nowadays is published in several issues each identified by a Thai letter and a number. The ones I only come across are the letters Ko Kai (ก) and Ngo Ngu (ง). The Ko Kai series seems to be for the higher level entities - provinces, districts and municipalities, while the Ngo Ngu is for lower levels - minor districts, subdistricts, villages, sanitary districts and the TAO. There are two further issues, Kho Khai (ข) which seems to be on announcements on the aristocracy only, and Kho Kwai (ค) for business registrations.
There are however also announcements in my database which have the additional word phiset (พิเศษ) in the issue identifier, for example the upgrade of the town Surat Thani to city status in issue พิเศษ 67 ง. This word means special, yet I cannot find a system what a special issue differs from the normal one, as it has the same kinds of announcement in either issues. For a totally different meaning of phiset, see this post in Rikkers blog on the Thai language.
Before 1995, the special issues had a trailing chabab phiset (ฉบับพิเศษ), for example the creation of minor district Viphavadi, published in issue=53 ง ฉบับพิเศษ. As "chabab" means issue, it then had explicitly "special issue" in the issue name.