Hello. I'm an "expert" at nary a thing, but understand that the T-Series (from the "T" prefix in the serial numbers) were classic Hi Powers manufactured during the '60's. There are reports of some being made in 1970 or so, but I'm not a historian and cannot say for sure. FN also excels at cutoff dates being "suggestions" rather than hard and fast fact.
Some Hi Power fans consider the T to be the apex rendering of the Hi Power with regard to fit and finish. Most that I've seen and shot had decent triggers out of the box and some worked fine with JHP while others balked. The T's do have the older humped feed ramp that can be problematic with other than ball.
They do not have the internal firing pin safety found on the Mk III (and late production Mk II) pistols sold in the US. They do have the magazine disconnect.
The T guns came with the traditional walnut stock and have the small fixed sights and the abbreviated, classic single-side thumb safety. I've seen none in other than bright blue finish and those all had the FN ring hammer.
Best.