Hi Powers use a date code system: Earlier codes; 1954 to 1964 = 70000 to circa 115823, '64 to '69: T136568 to circa T261000, followed be the introduction of the 2-digit year code and "C". For example, a Hi Power made in 1973 would have a serial number starting "73C…" and so forth.
T series are considered the best of the breed but one should not over pay. Only guns which are unshot or in LNIB are really collectible. The have ring hammers and smaller single side thumb safeties. Sights on these can vary from and smaller half moon sights, adjustable sights and even tangent sights. IMHO people way over pay for these all the time. They are called T series because of the T in the serial number. These came with a black leather pouch lined with sheep skin and true collectors want the original mags box and pouch.
There are also pre T series guns which have an internal extractor. Like this one.
This is a C series gun but has been customized.... C series guns are excellent and the majority have the classic deep blue finish. They can have half moon fixed, adjustable sights or tangent. Again these are called C series guns because of the C in the serial number. These have a spur hammer. The serial number is on the front of the grip. You can almost make it out in the picture above. For a classic looking Hi Power these are a better value than the T series.
The more modern MKII & MKIII Hi Powers use a date code to determine when they were made. Below is the key to FN dates of manufacture via the serial numbers. Newer Hi Powers are MKIIs or MKIIIs. They come in blue and matte black finishes again with adjustable and fixed sight models available.
Z = 1
Y = 2
X = 3
W = 4
V = 5
T = 6
R = 7
P = 8
N = 9
M = 0
These guns for the most part come with Ambi-safeties and spur hammers. MKIIs where made and assembled in Belgium. MKIIIs are made in Belguim but assembled in Portgual. They have more modern 3 dot sights.
This is a Practical MKIII
Most MKIIIs also have cast frames. All Hi Powers before the MKIII had forged frames. The cast frame is stronger than the forged.
Go on over to the late Stephen Camp's Site for more info....
http://www.hipowersandhandguns.com/HiPowerComments.html