Mine goes up to 11.
I cannot say that this is impossible, but it strikes me as unlikely.
The telegraph rings when the lever is moved to call the engineer from wherever he may be in the (noisy) engine room to read the dial on the telegraph. An engineer who only heard two of three rings would still find “Ahead Full” indicated when he arrived at the telegraph to read it.
I suspect that it is more likely that specific instructions would be handled by the blower/speaking tube. In other words, the telegraph would be rung to get the engineer’s attention. When he arrived at the telgraph, he would ring back, setting the same setting to acknowledge that he had heard and seen the telegraph instruction. His repeating of the telegraph instruction (causing it to ring on the bridge) would notify the bridge that he had arrived at that station and they would then use the blower to speak to him by voice to communicate the exact instructions.
This US Navy page shows a telegraph (presumably newer than the one to which Mangetout linked) that includes speeds of 1/3, 2/3, full speed and flank speed. (The secondarily linked picture is not as clear as the earlier link.) There is also the possibility that naval telegraphs had a flank speed where merchant marine telegraphs did not. The secondary link page (bottom row, far right) also notes that the telegraph had an indicator to display shaft speed, although it does not provide a clear photo of that device.
In our (relatively quieter) engine room when I was sailing, the officer in the pilot house used to specify by phone to the engineer the exact number of revolutions he wanted.
Regarding the shaft speed indicator, I am guessing that the three dark windows on the brass plate below the telegraph on the original photo are the shaft speed dials. They may not indicate the actual shaft speed, but a code to identify a predetermined set of shaft speeds. (I am guessing based on the (hard to read) information on the Operations Engine Manual from the USS Enterprise, CV6.)
I logged many thousands of hours at the throttles aboard a Guided Missile Destroyer (USS Somers, DDG-34) in the late 70’s, so I’ve got some experience with the Engine Order Telegraph (we NEVER called it that).
On my ship, the settings were BEM (Back Emergency), Back Full, Back 2/3, Back 1/3, Stop, 1/3, 2/3, Std., Full, Flank. Each setting had a corresponding shaft RPM listed on a chart at the throttle station. Additionally, there was an indicator which gave the RPM order from the bridge. In most cases, the ordered RPM matched the chart, but in some cases, such as re-fueling, the main setting would stay the same (generally 2/3), but the RPM setting would be changed every few minutes. it was easy to tell when a rookie was on the bridge during re-fueling as they would order changes of 1 RPM. At those times, I would generally just blow in the direction of the throttle wheel and wait for the next order for another 1 or 2 rpm. The engine speed indicator wasn’t really accurate to 1 RPM.
When an order came down from the bridge, the dial in the engine room would move, accompanied by a bell. The throttleman would “answer” by turning a knob to match the given order. The order was usually also relayed down by voice over a sound-powered phone circuit, to which the throttleman would answer “ahead 2/3, aye” or whatever.
In the case of a Back Emergency or Flank speed order, the signal would be sent down twice in rapid successsion, ringing the bell twice to get the enginerooms’ attention.
In addition to the repeaters in the enginerooms, there was a similar setup in the boiler rooms (but without the capability to answer back, to alert the boiler techs when an increase in steam was needed.
Oh yeah, the OP. MY ship didn’t have a “Dead Slow” setting, but the order could have been given by the RPM dial with a 1/3 setting on the speed indicator.
Slow speed ship handling is an immensely complicated topic. However, large ships generate significant turning force by the action of prop wash over the rudder, regardless of speed.
Oops, missed one! Between “Full” and “Flank” was “Fleet”. BTW, the forward speeds corresponded to a given speed through the water. From “Stop” to “Flank” it was 5, 10, 12, 15, 20, & 25+ knots, respectively. So “Dead Slow” would be something less than 5 kts.