My old Michel Ganzsachen-Katalog Europa West lists 2 items for the "10 ore over 12 ore" envelopes (U.5 and U.6).
The U5 is "blue to ultramarine" and is cataloged at 350DM unused and 350DM used. The U6 is "ultramarine to gray blue" and is cataloged at 2.50DM unused and 6DM used.
The catalog notes that there are various settings of the value tablet inscriptions on these first envelopes (U1 - U6). "The prices are valid for the cheapest type." Specialists are guided to Sveriges Helsaker by S. Lillehook, 1947 and 1950.
I might be able to puzzle out the Swedish, but I'd need to see the text, since I don't have the Facit catalog.
I do have the Ascher catalog, however, and my German is MUCH better than my Swedish. It appears that these two cards are listed by Ascher together as #4, where he differentiates five types, based on the types of envelopes #1 and #2 that were overprinted:
#4a: Overprint on envelope #1a (blue) #4b: Overprint on envelope #1b (grey-blue) #4c: Overprint on envelope #1c (Prussian blue) #4d: Overprint on envelope #1d (ultramarine) #4e: Overprint on envelope #2 (ultramarine)
Ascher distinguishes #1 and #2 thus: #1 has KONGL-POSTVERKET L. A. B. NKPG. embossed on the left side-flap, whereas #2 has a shorter inscription, KONGL-POSTVERKET, embossed on the right side-flap.
My guess is that Facit #4 corresponds to Ascher #4a-d and Facit #5 corresponds to Ascher #4e, but without seeing the Facit text, I can't say for sure.