off-hand, Id say, at LEAST......early-mid '70's.....certainly produced alongside the Big Boy.....maybe older, say the 60's?
the motor is the early style, in my eye's.......certainly an impressive loco when viewed from a normal modeller's distance.
Your example has certainly seen some action, judging by the wear on the wheelsets.
As to 'worth'?
well.I got mine for a couple of quid, back towards late 1989 or early 1990's.......promptly put it onto the NMRA modular layout and off it went........only 'fell apart' when I tried to give it a 'service!'
I also got a Mantua 4-6-4 with a stuffed motor, in with the deal.
re-motored with a second-hand Pittman motor.....those old Mantua's really did move with grace and gravitas??
Real olde-worlde diecast stuff!!!
all of which doesn't REALLY answer your question, does it?