SIG 5 says rod used with FT-172 is 12" long. And rod plus shock mount used with FT-178 is approximately 12" LOA. FT-172 anti-sway for BC-191 transmitter uses a Lord 150PH12 shock. FT-178 for BC-312 et al receivers uses a Lord 150PH6. The two shock mounts are the same except for load rating. And are the same size as the ones under FT-151 (which are not threaded, either). Neither description in SIG 5 mentions the slotted angle bracket and (backing plate for transmitter) shown in various TM 11-2700 series manuals. But that appears to be the way it was actually done as I've seen numerous BC-312 family cabinets with two four-hole patterns for the brackets. And some BC-191's with an 8-hole pattern.
TM 11-2710, 07/22/44. Installation of Radio Equipment in Truck, 3/4-ton, 4x4, Carryall, 58 pages. PB 23460 - BSIR 2(2):97; 07/12/46 (BEING SCANNED)