Orkney Image Library

It's not only the name but there are some more 'inconsistencies' ...
i) Prien sank only 31 ships - not 45
ii) he sank just 191,919 tons - not 211,000 (and damaged further 62,751 tons and not only 20,000 tons)
and - the most relevant of all:
iii) Prien died 7 or 8 March, 1941 when U47 was lost with all hands ...
iV) Prien therefore had absolute no connection with the "Geleitzugschlacht Paukenschlag" which lastet from January to about August 1942.
I'm afraid to say, the mug is a fake ... ;-)
cheerio

... bdw:
The iron cross on the other side shows the year 1939 ... but the The Knights Cross with Oak Leaves was introduced on June 3, 1940 only ... ;-)

For Prien, his actions and a quite realistic description of the decorations etc. see here:
www.uboat.net/men/prien.htm (in English)

My son, who bought the stein, and I are aware not all the inscriptions on it make sense in the Gunther Prien context; but rather than torpedo it as a fake we think it's the work of an ill-informed or propaganda-driven maker. In any case it's an unusual and interesting artefact.

Sorry, Ian, but I used the word 'fake' with regard to some ideas which came up as an answer to the first photograph. Since then I've talked to some other Germans and by now I'm sure that it is neither "a souvenir U-boat beer mug "celebrating" a subs success in operation drumbeat" (as Graham guessed), nor is it "Presumably a commemorative drinking cup from a German escort vessel" (as Neil said), nor is it "the work of an ill-informed or propaganda-driven maker" as you said - at least not of a 'propaganda driven German maker'. In addition to the already described mistakes I want to point to a last aspect/mistake: Despite other problems of the typesetting, please have a look at the word "Kapitänleutnant". The word has three different letters "t". Two of them are used in the right way (the middle "t" and the last "t" in the part "leutnant"). The first "t" (in Kapitän) is wrong. That is a "t", which should be used only in the beginning of a word. Have a closer look, please: that "t" is conflicting with the dot of the forerunning letter "i".
And that indeed makes your stein to an "unusual and interesting artefact" because it is another indication (like the wrong year on the nights cross and the swastica on the cross) that leads to a maker, an "ill-informed maker" as you said ... or a non German, at least a maker, who was neither familiar with the German details of the history of the German decorations nor with the sophisticated details about the use of a German set of types and letters in those days.
My friend and I we both agree with you that the stein was produced in the late 1940s or the very early 1950s (not sure about one set of types). We are also sure now - as far as you can be just from the look at a photogtaphy - that the stein was not designed by a German and that it was not produced in Germany.
And that indeed leeds us to the very interesting question behind the stein:
How was it possible that such a "memorabilia" could be produced so shortly after the end of the war and most probably produced by either an American or a British manufacturer, because the use of the swastica was strictly forbidden by the allied forces (and still is in Germany) but it never was in the UK or the USA.
War sells ???
Prien sells ???
Heroism sells ???
It is an unusual and interesting artefact, indeed, it is ... and with regard to the very special Orkney Germany relations (far beyond Prien and the Royal Oak) of which I'm well aware it would be a piece worth its special place in a museum or place like Lynnes Naval Base and along with a proper information about its background.

Thank you Wolfgang for taking such a close interest in this mug. The niceties of the typesetting are a bit beyond me, but I have to remark that every illustration I can find of the Knight's Cross with Oak Leaves has the date 1939 and, of course, the swastika. I'll post a shot of the bottom of the mug, in case it may be of interest.

In the 60s model kits of German aircraft produced in Britain did not have the swastika on the tail fin

Aircraft model kits usually contained a sheet of 'decals' for the model-builder to apply. Presumably, kits for German aircraft omitted swastika decals? (I never assembled a German model so I don't know.) No doubt this would be to allow sales in Germany.