, welcome to the forum! I encourage you to participate in the wide variety of conversations concerning language-learning here on the forum and, hopefully, share additional treasures from your collection.
As the Assimil 78 rpm records that you have in your possession are shellac
, which became a strategic material during WWII, and not vinyl, which was used as a substitute to the former, I am tempted to conclude that your set of records was pressed in the late 1930s to accompany the "Assimil Le français sans peine"
or "Assimil French Without Toil"
course. The Assimil recordings were unilingual (in the target language) and were available either for separate purchase or as part of packages which included the course book.
Although the Assimil course books from the 1930’s are becoming increasingly rare, copies are still available at reasonable prices via eBay, Amazon, and the like. Whereas “new editions” of the 1930’s courses were published every few years through to the early 1970’s, the contents (that is, the dialogues and the exercises) remained unchanged. Thus, the original audio recordings which you have in your possession would be valid for any of the editions right through to the 1970’s. The publisher kept pace with the changes in recording technology: the 78 rpm shellac records were supplanted by 45 rpm vinyl records, which were in turn supplanted by audio cassettes, thence compact discs, and so on.
As the audio recordings from the 1930’s are also available for purchase as vintage sets of 45 rpm vinyl records, or as audio cassettes and, as mp3 versions circulate freely on the internet, putting a value on the original 78 rpm shellac records would be difficult. There is presently one set of 45 rpm vinyl records for this course available for sale on eBay for approximately 85 GBP; however, prices vary considerably. I would imagine that your set of records would appeal mostly to collectors of vintage language courses. Should you be interested in selling your collection of Assimil 78 rpm shellac records, you might be able to increase their appeal to prospective collectors by purchasing the relevant course book from the period and by offering the recombined set of materials as a package.
Collectors, start your engines!
PS: To other vintage buffs, if I have erred, please feel at liberty to correct me.