the dual sync chassis were made at a time when many cabs required the facility to handle both 15khz and 25khz,also it made sense for manufacturers to make one chassis two options
the first dual syncs are switched using a jumper plug,if you look at the u5000,polo1 dual,nanao ms8/ms9 and sanwa pm1745 they all have a jumper plug-a tech could swap out a ega game for a cga and only have to move the jumper-you tend to find the nanao and sanwa in sega jap/export cabs,the u5000 in europe built sega and midway,the polo 1 dual in electrocoin stuff
these jumper plugs caused a few problems whereby a crappy tech would plug it in wrong and kaboom,this is where hantarex stole the show with their auto ranging dual sync thus completely eliminating this problem-at this point hantarex became the norm monitor used on u.k cabs(and because the u5000 caused so many problems)
as the trend tended to die away from universal cabs to dedicated then the manufacturers tended to install single frequency monitors like in the sega naomi cabs which used nanao ms2932 and sanwa 29es31.
the japanese bucked this trend and instead starting using the first tri sync monitors,this was mainly to allow capability of 15khz rather than 25khz-the first of which were switchable on the remote panel then later became auto syncing.
it was only in last years or so that the digital multi sync monitor have emerged,which are only really useful if you have a multigame system(like mame)