Here is a cover I acquired recently in an auction in New Zealand nicely franked with the three high-value denominations that appeared in 1935: the 10F+(10F) and 20F+(20F) Intellectuals and the 10F View of Vianden pictorial. Posted from the government's Postes et Télégraphes Office on March 7, 1936, to Erkelenz, Germany (near Cologne) where it was received the next day, the stamps would overfrank a 20 g registered letter to Germany in 1936 by 27F.
This is obviously a collector-created (or collector-requested) cover, but it is satisfying nonetheless, as I have yet to find a 10F or 20F Intellectual on an insured money letter or a parcel coupon for shipment of a sack of potatoes. Frankly, I doubt that either exists, but you never really know for sure. Prifix 2007 values covers of the 10F and 20F at 1750 and 2500 Euros, respectively.
As I explain here, the 15-stamp Intellectuals set was primarily sold to collectors to raise money to assist refugees fleeing Nazi Germany. Only 9,582 sets of these pricey semi-postals were issued. They were sold for double face value and remained on sale for just one year. And their postal validity was short-lived: March 1, 1935 to December 31, 1936. Scott refused to include them in their catalogs for many years, not relenting until their availability for postal use had been convincingly demonstrated.
The View of Vianden pictorial appeared on November 15, 1935, and was valid until the German occupation Hindenberg overprints appeared on October 1, 1940. Prifix 2007 values this stamp on cover as worth only 65 Euros. I would suggest that this figure is unjustifiably low. If you have a couple covers franked with this stamp, you are fortunate!
In addition to preparing first day covers of the Intelletuals, Maury Swartz used all of the Intellectuals on a series of mailings in 1935 to a Rev. Chaplain Hensgen in Luxembourg-Grund (Rev. Hensgen probably was the prison chaplain at the national prison in Luxembourg-Grund). Fred Wilmar, a collector in Moraga, California, was quite proud of these covers. In the 1970s when he sold the covers in a Richard Wolffers auction, I was pleased to add them to my semi-postal collection. Here are the 10F and 20F covers and the equally scarce (in my view) 10F Vianden:
Kayl to Luxembourg-Grund
August 9, 1935
Kayl to Luxembourg-Grund
September 30, 1935
Luxembourg-Ville to West Orange, New Jersey, USA
October 31, 1936, received November 10, 1936