Tobruk – Several of the soldiers interviewed in D DAY Through German Eyes - Wehrmacht Soldier Accounts of June 6th 1944, by Holger Eckhertz (DTZ History, 2015) referred to their bunkers as Tobruks. I could guess its etymology—from Tobruk in Libya, the site of famous battles during World War II—but couldn't visualize what kind of bunker it might be. Fortunately there are lots of images of tobruk bunkers in Wikimedia Commons, and a very informative site about the Regelbau architecture of German fortifications from the World War II era. Here's how the latter source defines a Tobruk:
The Tobruk or "ringstellung" is basically a reinforced foxhole, some with a small, two-man habitat attached to it. The simplest version is named Bauform 201 or 58c, but a a variety of bunkers emerged from it. Tobruks are also an integral part of many larger bunkers, where they serve as observation posts and machinegun positions.
Feldwebel – None of the German military ranks are translated in The Germans in Normandy, by Richard Hargreaves (Pen and Sword, 2006). Perhaps the author simply wanted to avoid having to choose between, say, private first class and lance corporal to translate Gefreiter or Sturmmann ('stormtrooper', the SS paramilitary equivalent). There is lots of variation across anglophone militaries, and especially across various service branches. But perhaps the author also wanted an easy way to signal the distinction between Wehrmacht (regular army) ranks and their Waffen-SS equivalents. For instance, an SS-Hauptsturmführer is equivalent to a Wehrmacht Hauptmann (Army captain).
One of the Wehrmacht ranks I was surprised not to recognize was Feldwebel 'sergeant'. (The same term has been borrowed by several other European armies, including those of Russia and Sweden.) It dates back to the early days of massed infantry tactics that required careful alignment of troops wielding pikes or firing muskets. The Feldwebel was the person who kept the troops in the field properly aligned.
German Wikipedia says Feldwebel derives from Old High German weibôn 'sich hin und her bewegen' ('to go back and forth') but doesn't cite a source, and translates Webel as Gerichtsdiener ('court usher'). The Swiss German rank is Feldweibel, related to Weibel (also Amtsweibel or Amtsdiener), the officer in charge of protocol in various official gatherings.
English Wikipedia cites the same Old High German etymology but translates Webel too simply as 'usher' (as in court usher, Gentleman Usher of the Royal Household and of various anglophone parliaments, or White House Chief Usher). If I had to put a contemporary label on all these formal order-keeping roles, I would lump them into the category of sergeant-at-arms, rather than usher. (It's ironic that "sergeant-at-arms" now distinguishes various sorts of civilian order-keepers from military order-keeping sergeants.)
French Wikipedia gives Feldwebel a slightly different etymology (also without citing a source): "vieil allemand waibel, pièce de métier à tisser servant à ramener tous les fils sur la ligne (peigne)" ('Old German waibel, the loom piece serving to keep all the threads aligned [comb]').
The last etymology seems to me to get closer to the source of the term Webel, a Middle High German cognate of English weft, according to Guus Kroonen's The Proto-Germanic n-stems.: A study in diachronic morphophonology (Rodopi, 2011). The weft threads are those that go back and forth ('sich hin und her bewegen') across the warp threads to weave fabrics on a loom.
This reminds me of the first line of the first dialog I had to memorize when I took the Romanian language course at the Defense Language Institute in Monterey, California, in 1969: "Bună ziua, Domnule Locotenent!" ("Guten Tag, Herr Leutnant!")