Staggering of arms. Many railroads that used color-light signals distinguished automatic signals from interlocking signals by staggering the arms. E.g., if the signal face on the upper arm was to the left of the signal mast, the middle arm was to the mast's right, and the lower arm to the left again. In contrast to automatic signals, interlocking signals on these roads were installed with all signal faces in a vertical line. Significance of number plate.
Number plates are significant on most railroads.
These are used to identify an automatic signal, and to distinguish it from an interlocking signal. The signal-numbering scheme varied from road to road but was frequently based on the milepost. A common scheme was to use the milepost, including tenths of a mile, times 10. That same signal would be numbered 182E, i.e., milepost 18, track 2, eastbound. Significance conflicts For a number of roads which used number plates and staggered signal arms, it is not at all clear from the rules which of these 2 items is, or if both are, significant.