I think my powers of investigation are slipping. I was so happy that I found the box for the AHM trolley I forgot to open it to see if there was anything interesting inside. It turns out there was.
The data sheet was right where it should be: tucked inside the bottom of the plastic insert used to hold the trolley in place.
And maybe more importantly was some code that was stamped on a box flap. It looks like April 1975 was part of the code, but I'm not sure what the rest means. So, maybe it wasn't released in Feb '76, but earlier in '75. The signs so far point to sometime in '75 or '76, so maybe I bought mine a little earlier than I thought.
Oh, and one other thing, Vince suggested I look into the work of David Voice. I'm ashamed to say I hadn't heard of him, but I found his book for $5 at an online book seller, so I ordered it. It's quite a charming look at tram modelling circa 1982. Interestingly the first kitbash project it discusses is a conversion of the AHM trolley to a British prototype. From the text, it looks like the Birney was manufactured by a company called Mehanotehnika, now called Mehano, located in Slovenia. So, this lines up with the box stating the model was imported from Yugoslavia back in the '70s. Well, I'm off to see what other tidbits Mr. Voice has for me :-)