The 1965 edition is one of the 6 sought after TV 21 Extras (or Summer Specials). The 1965 Summer Extra edition is the second of the six in fact. This special is pre-Thunderbirds which didn't air on television in some areas until 30th September 1965 and nationally from December.
The 1965 edition is genuinely scarce. They turn up on eBay UK once in a seldom while. The 1965 Extra has a free gift of the Cosmic Capers Kit - adds about £100-£150 to the value if absolutely unused, less so if there is wear or creases from opening and closing.
The 1966 Summer Extra is probably the most available/common of all these specials. It was first advertised for sale in TV 21 #79 dated July 23rd 1966. This one has Thunderbirds in black and white and one story in colour plus Special Agent 21, The Munsters and features and illustrations of future combat vehicles. Have a look at the Jungle Spider Troop Carrier - looks straight out of Star Wars!
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.