Was Use of Postal Stationery Cutouts Ever Permissible in Luxembourg?

We occasionally see cutouts from postal stationery used as postage.  I’ve always been uncertain whether this practice was permitted within Luxembourg (and within the UPU mails generally).  In Great Britain, for example, the postal use of cut-outs was banned under the Post Office Act of 1870 when wrappers and postal cards were introduced. This prohibition remained in force until 31 December 1904.  And the current United States Postal Service Domestic Mail Manual at 604 Paragraph 1.3(d) provides, in part, that stamp imprints cut from stamped envelopes, aerogrammes, or stamped cards are invalid to pay postage for U.S. domestic or U.S.-originated international mail.

But what about Luxembourg?

Here are three examples—one from 1892, another from 1915, and a third from 1935.  Only one was taxed:

5-centime Green Allegory Postal Card Cutouts 
pay postage on a UPU letter
to Switzerland
in 1892

 

 

25-centime UPU Letter
Luxembourg-Ville, 21 Aug 1892,
to Bern, Switzerland

 

 

 

5-Centime Arms Postal Card Cutout
pays postage on a Letter to France in 1915
from Carmelite Nuns in Luxembourg
with the Cutout use taxed

 

 

Luxembourg-Ville, 5 Feb 1915?, on a 10c/20g letter to Paris, France,
with reverse cachet of the Carmelites Tertiaires, Luxembourg

 

40c Charlotte Postal Viewcard Cutout

Pays Postage on a Domestic Advertising Cover

in 1935

 

 

Luxembourg-Ville, 17 Oct 1935, to Limpertsberg

at the 75c Domestic Letter Rate