If it's about 2" wide and very thin, I think it's a pocket chisel. These were for chopping out the "pockets" in the boxes of sash windows, which gave access to the sash weights. It would have been about twice as long originally.
Sorry, I just noticed the picture with the tape measure - five inches wide!
I think it might be a rare Roller Coverer's Chisel!
Not many people will have heard of that. I've never seen one in the flesh, but I did spot it in an old 1928 Marples catalogue. Intrigued, I asked the Tools and Trades History Society. Nobody responded, but I found what I believe is the correct explanation. You can read everything I know about it here:
I hesitate to jump in here - really NOT an area of my knowledge - but watching the BBC TV programme "The Repair Shop" from time to time, I THINK I see leather expert Suzy Fletcher using a similar-looking tool for cutting leather.