Sako used two arrangements for their "Hi-Power" rifles.
The early one was exactly the same as the late HVA and the other was the Surpeme action, assembled with the then new Sako trigger.
Sako used these actions from 1951 to 1960, but they continued selling their remaining stock well after the introduction of their L61R action in 1962.
Serial number for the Hi-Power serie started at 100001.
I have a "late" arrangement, and Trade Ex have (had) two of the early arrangement on their website.
264 Magnum,
HVA did not make the "Hi-Power", or, Model of 1951 with beech stocks. They were all walnut. The 640 wich was fitted with both Walnut (mostly for export) and beech.
In reality, both "Hi-Power" and 1951 are unofficial denominations, made out by Tradewinds and does not exist in any HVA publications, at HVA they were intended for export and were called 1000, 1100 and 2104 and their serial range is mixed with the 1640 of the same time period.
Basically, they were the same as the 640 line, but with better finishing and selected wood, sharing stock design with the 1640 line.
Some picture for comparison;
Left is a Sako-Hi-Power, a HVA 3000 Crown Grade, a Brno 21H, a M/649 (M/38) and a M/46 (M/94)
Sako Hi-Power (late, 1956, on a Supreme Action) and a late HVA M/648 made on a post 1949 action (rifle made in 1952)
Sako Hi-Power on a FN Supreme action, bolt shroud and safety details.