Great news!!! Found the answer when Das Diadem & Der Goldene Schlüssel were shot and moreover...
Anyone who thought that these films were made later than 1966 were.... Right.
Well, really, it’s hard to imagine that they were shot in 1966 in West Germany, at the least due to facts that the first Avengers episode (with Emma Peel) was transmitted in West Germany on October 18, 1966 and the German television channel ZDF showed only 5 episodes in 1966. It turns out that the Germans were so impressed by Emma Peel that literally only after one or two-three episodes on the German screens did they offer Diana to star in Germany with their money?
Article from
magazine Bravo (№44, 28 October 1968), where says the following:
http://deadline.theavengers.tv/schirm.htm
Wem das nicht genügt, der kann sich jetzt seinen eigenen Emma-Film "für den Hausgebrauch" kaufen: einen Acht-Millimeter-Schmalfilm, der in Duisburg und Mülheim/Ruhr mit Diana gedreht wurde. Er dauert fünf Minuten und heißt "Der rote Schlüssel". Zehn solcher Kurzfilme für jedermann mit Diana sind geplant.
translation in english:
If that is not enough, you can now buy individual Emma-film "for home use": 8mm movie that was filmed in Duisburg and Mülheim an der Ruhr with Diana. It lasts five minutes and is called "The Red Key" (my comment: actually, The Golden Key). Ten such short films for everyone with Diana are planned.
Original of this article from Bravo here:
https://cloud.mail.ru/public/5Mk4/UgryE1M31
Note from the
newspaper Hamburger Abendblatt (28 August 1968) informs about:
translation in english:
The new series will be shot on film.
Hamburg, august 28
Diana Rigg, "impact force" Emma Peel from "The Avengers", wants to penetrate the new film series into the living quarters of German citizens. Her clever manager Jerome Banks decided not to pinch his Star on television and gave permission to copy the ten-episode series to Super-8 film so that any movie projector owner could watch it at home.
Filming of the series will begin in early September in Mülheim an der Ruhr. Emma, who prefers theater rather than television or cinema, persuaded the manager that she would play her part by imitating facial expressions. The first films will appear on September 28th by "Photokina Köln". All episodes are individual films and can be shot in color.
Original of this article from Hamburger Abendblatt here:
https://cloud.mail.ru/public/4GaY/2fixB6mEB
and finally, note from back cover of
magazine Bravo (№3, 13 January 1969) here, translated in english:
What she has learned as Emma Peel, Diana Rigg proves in her new film: the handling of heavy weapons and heavy boys. The film was filmed at Schloss Hugenpoet in Kettwig/Ruhr. If you like Diana Rigg for your home theater, you can buy the movie in the photo shop soon.
Result:
1. Summer news of 1968 said that a 10-episode film series with Diana was planned for release at 8mm, starting since late September.
2. Filming the episode "Der Goldene Schlüssel" took place in September or first half of Autumn (and maybe bit later) at the least in Mülheim an der Ruhr, Duisburg, and Schloss Hugenpoet in Kettwig.
3. Diana personally wanted to play her part by only imitating facial expressions.
4. This was planned for release on "Photokina Köln" initially.
5. Der Goldene Schlüssel was on sale already by Autumn 1968 (maybe already since late September).
6. Das Diadem was on sale no earlier than 1969 (mostly likely since Winter 1969).
Schloss Hugenpoet
https://de.wikipedia.org/wiki/Schloss_Hugenpoet