Is there a preferred traditional steel among the Finnish makers, and how hard do they run it?
Traditionally common man's puukkos were made recycling steel from sickles and files, after the forging of laminated blades was dropped in the late Middle Age (though Scanidinavian smiths have kept it to this day). In the 19th century came silversteel, but it's unclear what type of alloy it had at the time, since the name was used in different courties to identify different steels, Silversteel used today is Bohler K510.
0,7-0,8% C steels were qite the norm for working puukkos, while silversteel was more common in higher end ones.
Currently the most used steel is probably 80CrV2, a ThyssenKrupp circular saw steel that can be used very efficiently both for longer and shorter blades, changing slightly the tempering temperature. It's very easy to heat treat, has a great balance of resilience and edge holding and is quite cheap. If tempered under 60 HRC has a resilience and edge holding comparable to same hardness 1075, when tempered higher has an edge holding comparable to 52100 at the same hardness, but with just a bit more resilience. On puukkos I've seen it giving its absolute best at 62 HRC, allowing it to have an excellent edge holding while still being tough and even at this ahardness is very easy to touch up and resharpen.
K510 is mosty used on Tommi puukkos and has a very good edge holding, but lacks resilience. Other carbon steel used are C75, O1 and 52100. Martti Malinen, mentione dabove has specialized in the sforging of 52100 and has used it for 25 years now. Some smiths, like the mentioned Henri Tikkanen still use plenty of recycled steel, in his case a 0,85% C steel from plow blades and a 1,3% C steel from Viiala files.
As for the hardness, evey smith has his own prefernce, but I'd say that currently is unlikely to find handcrafted puukkos tempered under 58 HRC and over 64 HRC, the majority settling for somehing in between 59 and 61 HRC.