I acquired this cover a couple of days ago. It was registered and posted in Tallinn 16 September 1925, transited Helsinki 17 September 1925, and received in Oslo 21 September 1925. The adhesives used are the 1924 15m on 5m airmail issue, full set (perforate) of the 1925 airmail issue and two 20m "blacksmith" stamps to cover postage.
The 15m on 5m stamp covers the airmail fee and the 1925 set is purely philatelic. The two airmail cachets in red on the front were in use between 1923 and 1927. On the rear is a bilingual Finnish and Swedish "by airmail" cachet. However, the transit time of 4 days suggests that it may not have eventually been carried by airmail, as other similar covers only have an additional day's transit time. Maybe it missed a connecting flight and had to be carried on a later flight?
Here's an esoteric question: does anyone know the frequency of airmail flights between Finland and Norway at this time?