I am not that familiar with Hi-Powers and just searched the number on line. That is only half of the number since it has two sets the first part is 245PP then a space followed by 76XXX. I have not been able to show what the 245PP stands for yet. There is a gun show in Raleigh this weekend and I may be able to find a book on the Hi-Power.msgt, I believe something is amiss with your dating of your HP providing you have pictured your's.
IIRC HP s/n's aren't etched in stone and there are some discrepancies leading to confusion for most folks.
The HP pictured has an external extractor instead of a controlled feed internal extractor. These were transitioned to around 1962. Also it has a spur hammer. I believe that was only a later trait but don't have a time frame to reference. I know my MKIII has a spur hammer but earlier C and T series HP's have Rowel/Commander style hammers.
It's also entirely possible you have a '50's era frame with a newer hammer and slide assembly.
I wish I could tell you where to go to get a definitive date for your pistol.