Alan, you are correct and it makes all the difference! Historisches-marine archiv.de has OLtZS Gunter Zehner as 2WO of U 415 which was sunk as a result of hitting a mine.
Colin, all is not lost.
Reg

Hello Colin, If you google" U 93 Interrogation Report" you will find it contains the crew list. Sorry but there is no Gunter Zelmer. These Reports contain alot of information including crew lists.
Reg Gleave

These two marks are illustrated in UK Censorship WWII, Chapter 4, page 163. There is said to be a third variety but I have never seen it.
I have assumed that the variety without printers imprint is Daynes RNL 101.
Most of the few I know of are associated with the Anglo Saxon Petroleum Company's tanker ANADARA when under repair in the south west
Reg Gleave

Hello Ingo,
I showed your email to Colin Baker, the co-author of the book. He lives about 500 metres away!  He could see nothing wrong or unusual with the envelope you show. He thinks it is the most common variety.
They are working on a new edition which should come out next year. He has given me a draft of the page covering these envelopes which I will send to you if it is of interest. The descriptions have been expanded to clarify points raised by collectors.
Best regards  Reg Gleave

Googling Aerial Employment Company shows a piece from the Harrogate Herald of 31 October 1917 referring to a Private E Williams attached to an Aerial Employment Company thanking the paper for a watch. Sadly there is no more information but it seems to confirm the address.
Reg Gleave

Hello Tony,
I think the address is EYNHAM ROAD, ST QUINTINS PARK.
It is close to Hammersmith Hospital ( & Wormwood Scrubs Prison )
But that doesn't get you very much further.
Reg

The catalogue illustration of the "1916" version has a poor strike of "post early in the day"
My 1983 edition of Collect British Postmarks does not include this before 1937.
Reg Gleave

Can anybody tell me anything about the Spanish WW1 internment camp which was at Alcala de Henares which is to the east of Madrid or suggest where I can find anything. It seems to have been used to hold German sailors from damaged submarines which had made it into Spanish waters.
Reg Gleave

The article was published in the Jan/March 1990 edition of POSTSCRIPT, the newsletter of the Society of Postal Historians.
Bob Roberts died in the Spring of 1991

I you google HRC Blagden there are several references to his involvement with the filtration works for supplying Alexandria with potable water. There is even a reference to a paper given to the Institution of Civil Engineers and you cannot get better than that!
Reg Gleave