They possibly built it that way for broadcast (1950s), maybe also to cover up some circuitry noise (hum)? Possibly also aged capacitor.
Was thinking near zero impedance (e.g. THAT1646) feeding directly into the input transformer to test it.
Benefit of 1512 in front is cheap and easy additional gain, if the circuit can handle it. And could be used 'clean' in other setups without 'colour' from that mixer patched after it.
But I agree, it would reduce that mixer to a mere 'colour box' patchable only when desired.