Not shown on circuit schematic as it is
not user replaceable. Since we have submitted multiple audio products (consumer & pro) for UL/CSA certification we know this is a mandatory requirement, as along with the unit for certification we have to supply (5) pcs of the power transformer.
Can't really comment on Quad amplifiers but to my knowledge this also required for CE approvals as well, but if the amplifier is older it may predate this requirement..
Just my $0.02...