Thom,
I Googled Riihimaki & checked out some of the images available. From those, it seems that your action is the Sako L46, the precursor of the L461. According to De Haas, it was made with a single shot variant with a pinned plate blanking the action's magazine cutout, though that would be a simple custom modification in itself. It also had a clone of the M70 Winchester trigger, rather than the morticed falling sear of the later L461 & a square bolt shroud with a safety on it.
I suspect that once used for a custom rifle, it would be less attractive than an original rifle to Sako collectors.
John