Michel reports that catalogue number 1052 comes in two perforation sizes. The standard line perforation of 12 (1052A) and a much more coarse line perforation that varies between 3 and 4 (1052C). The latter is known to have been used in June of 1979 which means 15 months prior to the L12 version being released. Michel also lists a rouletted version of 1053 (Flag of Vietnam) as 1053D. The mint version of the 1052 C is listed at only €6 but after years of looking for one I have pretty much given up hope that I will ever find one. The reason probably has to do with the circumstances the perforation variety came into existence.
The entire set of Michel 1052-1054 was produced locally at Tran Phu Printing House and it apparently had been plagued by a host of production problems. This is evidenced by the relatively large number of errors that were incurred during production. Missing back grounds, inverted or missing colors and perforation errors can be found with relative ease. What is not clear is how much of those were released by post offices or through dubious sources as printers waste.
At any rate, it is entirely possible that some sheets of this stamp were released by error in the imperforated version to one or more post office. In order to allow for easier separation they were then submitted to a local perforation process by sewing machine which resulted in the L3-4 version. There is no other rational explanation as the main production office would never have used such a wide and impractical perforation given the available perforation equipment.
Below is a postal use letter that I purchased from a French dealer. It was mailed in a small post office outside Ho Chi Minh City (Zip Code 15152) on May 25, 1979. He sold the cover as “imperforated stamps“, however if one looks closely, one can clearly see the L4 perforation of Michel 1052C. It must not have worked very well as all the remaining edges of the stamps have been cut by scissor as this apparently was easier for separation than using the coarse perforation.
The sewing machine cancellation can clearly be seen between the two stamps
My guess is that most of these stamps were mostly used up in the post office(s) they were produced. Since collectors usually shop at the large post office it is unlikely that someone would have picked up many mint versions of 1052C for collection purposes. Nevertheless at least one mint stamp of that perforation must exist as Michel’s policy is not to list a stamp unless it has been submitted to the editor in the original. However equally clear is that the Michel price of €6 for the mint version is rather ridiculous. Check your holdings of mint and used stamps for tho variety, however my guess would be that you most likely find it on a postally used cover.
I would be curious to learn from readers if they have copies of 1052C in any version or the equally elusive 1053D (rouletted).