The Finns did not start using the "SA" Army acceptance stamp until late 1942, at which point it began being used on all new production rifles. The "SA" stamp was then added to most previously produced rifles over time, once they cycled back through an arsenal or weapons depot. But there's a decent number of previously produced rifles were not restamped for whatever reason.
In addition, it's not uncommon to find Finnish Civil Guard rifles that are not stamped "SA", since it's an Army acceptance mark. While most Civil Guard rifles were transferred to the Army during the Winter and Continuation Wars when rifles for the military were in short supply, some Civil Guard rifles were never transferred to the Army.
So Finnish rifles without "SA" markings could be Army rifles that were overlooked, or Civil Guard rifles that were never transferred to the Army.
S18708 corresponds to the Salo Civil Guard district.