suitbl fr use solely/prncplly wth apprts of hdgs nos to othe parts of hdg to eqpmnt fr rlwys, trmwys,roadwys,wtrwys etc(othr than those of hdg nos. ) elctrc sound/visual signllng apprts(e.g. bells sirens etc.)othr thn those of hdg no. / elctrcl capacitors fixd,variable/ adjustable(pre-set) elctrcl resistors...