U-Boot-Flagge

[olpe]

Hallo,
ich erhielt nachfolgende E-Mail:

Dear Sir or Madam,

A few years ago I purchased a WW 1 Imperial German Flag from a WW2 Veteran's son. The son stated that his father told him this flag was in a museum in Germany (the father was an American solider during WW2) and was adamant that it belonged to a WW 1 U-boat. Subsequently I bought the flag and not necessarily the story. However, after looking closely at the flag there appears to a museum stamp on the flag towards the edge of the worn side. My question is.... Did the WW1 U-boats have specially sized flags? The flag is about 25.75 inches tall and 35inches wide (from what remains). The stamp markings are as follows: "J" or "U" and 2624 or 2842 below that is a faint "U" or "O" followed by the number 20 or 28. Do these markings mean anything? I just want to put this to rest. I have attached some picture for your review.
Thank you for your consideration,
Bruce Beck

Kann dem Mann geholfen werden ? Würde mich sehr freuen ...
Unten die mitgeschickten Bilder der Flagge (falls der Bildinhalt des ersten Bildes der Restriktion unterliegen sollten, bitte entfernen ... )
Grüsse
OLPE

[Elo]

(falls der Bildinhalt des ersten Bildes der Restriktion unterliegen sollten, bitte entfernen ... )

Warum das? Das ist die Dienstflagge des Kaiserreichs. Das ist nichts verbotenes.