i have some info on neumann v475, also a summing amp with obviously quite similar topology. i suppose most of the "later" summing amps like v475, filtek, lawo v975, delta... etc. have "zero ohms" summing topology, so it may also apply for your unit. zero ohms means a very low i/p impedance, convenient for summing lots of sources (100 or more) together. gain is set by external series resistors (5k8 with v475 for unity gain IIRC), maybe an external feedback R in some cases.
a neumann service tech once told me that the i/p xformers were especially designed for 0 ohms circuits and don't work well in other applications. the o/p trannies might be more universal, turns ratio probably something in the 1:1 region.
but watch out! i once bought a later lawo line amp, the trannies of which i was going to bastardize, only to find out that they were actually "ACTIVE" xformers!! - after removing them from the board & some dissection, an active circuit with a driver IC appeared inside the trannie housing, with the actual transformer being much smaller =(((
HTH,
volker