The transformer feeds a block with a triangle symbol, so my hypothesis
is that this represents "rectifier" and it's a full-wave bridge,
especially as it says 42V= (42VDC) underneath. The block on the left
with three inputs at the top could be a three-phase rectifier, and the
small blocks with three terminals could be silicon controlled
rectifiers. Your mystery block must be something which flips into one
of two positions.

Dave W