Mixed Key Signature

Those who have studied extensively the organ scores of Baroque composers are quite used to running across key signatures which at times, compared with the actual key in which the music is meant to sound and our modern way of notating the key of a work, may be lacking a sharp or flat in the key signature.
Baroque pieces scored in the key of g minor, for example, may only show one flat (Bb) in the signature with the composer writing in all the Eb accidentals each time -- other pieces scored in the key of d minor may not show a flat in the signature at all with the composer writing in all the Bb accidentals each time -- and so on.
But ... what in the world is THIS ? ... (photo)
A key signature like this is something an organist also may run across, albeit much less often, in modern musical notation and is called a "custom signature," or "mixed signature."
Actually, this is no printing error -- it's simply the case of a modern composer not writing in any key per se and merely bending the rules of notation to lessen the work involved in writing in all the accidentals each time-- the music simply has a Bb and an F sharp and works as any other signature, i.e., every B is flat unless altered, and every F is sharp unless altered.
On first glance the key signature of this passage suggests g melodic minor but ascending only, since descending g melodic minor would specify an Fnat and Eb.
This signature (photo) isn't an exact fit with any of the modes either -- but if it were so described the mode would be Bb Lydian with an altered (augmented) 5th.
This same opening passage of music, if notated in the anticipated way these days, would be printed in the key of g minor, i.e. with 2 flats in the key signature, and any accidental F sharps, being raised leading tones, would be written in.