Could someone please explain to me exactly what this is ? and which cars would have had it ?

Well... a long time ago, I had a 69 vette that came with one, stock.

It's basically a switch box, the precursor to a capacitor discharge ignition. Rather than the point carrying the full coil primary current, the points turned the transister on and off, and the primary current was carried by the transister.

It gave a hotter more stable spark.

Kent, Here's an hours worth of reading for you.....

http://www.coloradocamaroclub.org/trnsignrevision4.htm

All you ever wanted to know, well almost.

Just the information I was looking for, thanks for the response.