I picked this up today. Can anyone tell me approximately when it was made?

I can see it's a very early one. Was it by any chance available alongside the first Sony Walkman (TPS-L2) in 1979?

Nice find. I have one NIB. Dont believe this was available in 79 or 80 alongside the TPS-L2 but probably 82-88 ? Here is good link in S3G on tapes
Cassette Tapes

Must be late 80's as its a c-100.
c-100 was not available in the 70's.