Does anyone know .... if the Langevin EQ-251-A and the Altec 61A passive
equalisers (but equalizers in US) are virtually the same thing,
i.e., same circuit but different manufacturer?
If so, DAK which firm designed the original circuit?
If not, DAK how do they differ in spec, apart from the mechanisms of their
slider step-attenuators?

These goods are at least 35 years old, each mono metal-screened module has
a rear, in-line protruding tongue, with gold ways of about 5/32" pitch to
sit in an
external s/s or d/s edge conn,
the Langevin has a keyed 5-pole edge while the Altec uses a full 6-pole
edge,
both using only 4 poles as named on internal pcbs: Com (2 poles), O/P, I/P,
and Ground.
The chassis poles are not compatible between them, using #6 (5 is blank),
and #5 respectively.

Either unit box is 1-1/2" wide, 3-3/8" high, 5-3/4" deep.
The archaic black fascia basically has
a 360 deg rotating switch with 2 shelf LF freq @ 40,100 points and 2
intermediate Offs
and
a 360 deg rotating switch with 4 bell mid-high @ 3k,5k,10k,15k points -
marked in KC,
both knobs turn a long shaft juggling several cam-operated leafs to various
gold-flashed pcb nodes (clever!?),
each selector has its dedicated max +12dB Lift /0 /max -16dB Cut, vertical
slider 2dB/step attenuator.