http://mirror2.ipdb.org/files/1176/Williams_1986_High_Speed_Instruction_Manual.pdf
Check page number 34 in the manual (page 40 of the .pdf file) for a diagram and list of the flashers. The two at the top corners, shown as #22 in the diagram, do not have plastic covers per se, but the one in the upper-right corner is actually hidden under a playfield plastic - at least on my High Speed.