Finnish Stitch 1+2½  (Pyhäjoki Stitch)

South Carelia, Kymenlaakso, Pyhäjoki, Vimpeli, Tervola (Finland)
Kannas (Carelian Isthmus, former Finland; area belongs to Russia since 1940)
Morkarla, Södermanland (Sweden), Sorunda 1

Hansen's Notation
UOOo/uUUOO F1 (or F2)  (video)
small o ja u = split the yarn with the needle (ie. needle in between plies)

Leinonen writes in her Master's Thesis (p. 54), that this stitch type (FS 1+2½) has been used in Finland, in Etelä-Karjala, Kannas and Kymenlaakso. 

Finnish Stitch 1+2½, (Pyhäjoki Stitch)
front side

Finnish Stitch 1+2½, (Pyhäjoki Stitch)
reverse side

In autumn 2010 I interviewed on the phone an older nalbinder (born 1930's), whose grandmother (born in Savo (Finland)) used to split one of the loops behind the thumb with the needle. Unfortunately this nalbinder didn't remember the exact stitch type her grandmother used, so it may have been for example 1+2½, 2+2½, or some else with a split loop. The nalbinder I interviewed, had later learnt another way of nalbinding (Finnish Stitch 2+2, without thumb loops (flat)).

The information about this stitch having been used in Pyhäjoki (Finland), is based on a blog 1/2013. I will add later some more information.

Split loops visible on the reverse side.