SIGNAL UNITS
There are three main symbols that indicate a signal unit:
the Signal Unit flag, a battery insignia, and communication
lines. You will find any combination of these on Signal Unit
cups. Below are some examples.

More examples are here:
Signal Unit page 2.
To the right are the relevant kanji:
DEN-SHIN

Below in combination: HIROSHIMA
DENSHIN-TAI (Hiroshima Signal Unit)
Here are 2 more important kanji: MU-SEN (Radio [Unit])
Below in combination: MU-SEN DENSHIN (Radio Signal Unit)
Written here is MU-SEN DEN-SHIN-TAI. Wireless Signal Unit.
2nd Regiment Signal Unit tray
Another nice example. The branches are
laurel, I think. 'Hiroshima Signal Unit.'
Inscribed 'Imperial Guard Signal Unit Regiment,
Discharge Commemorative'.
2nd Regiment
2nd Regiment
Rare Signal Unit collar tab cup. This collar tab has the battery insignia.
Inscribed 'Central China Dispatch, Signal Unit Commemorative'.
Signal Unit cup with the unit flag, the battery insignia, and
communication lines.
Inscribed 'China Incident, Victorious Return
Commemorative, Signal Unit 1st Regiment'.
Very cool Signal Unit cup with a dramatic flash of electric energy
shooting out of the Army star. Also communication lines and on
the reverse is the Signal Unit battery insignia.

Inscribed 'Signal Unit'.
The Signal Unit battery insignia is in the
center. Inscribed 'Dispatch 34th
Commemorative' and on the reverse
'Showa 3 [1928] Victorious Return'.  
Inscribed 'Signal Unit Commemorative'.