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I thought all the Soviet conversions to tandems were for training (which they did themselves) - the open second cockpit with MGs may have been for AG training.
The second caption basicly states that it's a two seater, back shooting, probably about 1942.
However, I've discovered on another forum that BW948 was an artillery spotter.
Will work on the first caption later - transcribing polish is tricky.
It's from Two seater Hurricane so don't take it as gospelbut they've got the same picture posted up.
I have a Hurricane II electrical and mechanical manual. Maybe it will shed some light but I think this is a soviet special like you said.
Is that the RAF Museum Series book - with all the Mk II manuals? I've got that too, and as far as I can remember there isn't any info on this.