Finland received German Stalhelms during the Winter War (1939), WW2, and into the 1950’s. The Finnish military used stalhelms through the 1970’s (source: https://www.warrelics.eu/forum/restoration-refurbishing/german-ww2-helmet-restoring-678211-2/#post1760959) and were kept in inventory as late as 2006 (source: https://ww2aircraft.net/forum/threads/cache-of-german-helmets-found.35289/#post-969658, https://www.varusteleka.com/en/product/finnish-m55-steel-helmet-surplus-unissued/69381 [product page]).
M1916:
Extra hole might be the furthest back one that is closest to the raised vent. 
This is the version of the Stalhelm Imperial Germany issued during World War 1 to reduce the amount of head injuries from explosives in trench warfare (source and further history: https://www.ir63.org/stahlhelm.html). The particular example shown here likely had an extra hole drilled into it some time after WW2 for an early version of the Finnish liner, itself a close copy of the original WW1 German liner, during the refurbishment process. This helmet was bought through Varusteleka in August 2020.
Inside:
Helmet swivel.
Three-panel liner like German WW1 liner. This is the earliest version of post-WW2 Finnish Stalhelm liner (source: https://www.warrelics.eu/forum/finnish-militaria/finnish-helmet-liners-787558/#post2151601). The original helmet swivel, originally for the German pickelhaube chinstrap (source: https://www.ir63.org/stahlhelm.html), is still used.
Chinstrap Buckle:
Top:
M1935:
This helmet is the more simplified version of the M1935 stalhem made during WW2 with punched ventilation holes for faster production (source: https://germanhelmetvault.com/learning-the-basics/). Collectors sometimes call these helmets “M40” or “M40/55” specifically for the Finnish ones. Finland acquired approximately 50000 Stalhelms from West Germany during the 1950’s (hence the “55” designation, source: https://www.warrelics.eu/forum/finnish-militaria/finnish-m40-m55-794474/).
Top:
Inside:
Helmet’s lip rolls inwards which is why it is an “M40” type. Liner was made after the war to refurbish the helmets in Finnish service.
Chinstrap has a protective layer of leather on face side where buckle is.