Hi Bruce,
That is definitely a decoder, but not an Armstrong one. The component style and layout is very reminiscent of my
Goodmans Module 80 stereo receiver which dated from about 1974.
PC619A also has a very Goodmans ring to it.
Sadly, I don't have the unit or the manual any more, but the stereo beacon was certainly an LES lamp just like that. There were a further two which formed a crude but effective tuning indicator (tune for equal brightness).
Some pics here but I can't work out how to enlarge them, even though I am (was?) a member:
http://www.radiomuseum.org/r/goodmans_module_80.html
Goodmans by then were part of BRC so it may have been used in Ferguson equipment too.
N.