On first glance, it looks very like my first dinghy. Merlin 564. My Dad bought it for me in about 1972. Long story. Sailed it once, and eventually met John Oakley (read on).
It turned out to be very rotten, so decks were off before we found that seven planks, keel, hog, and half the transom were too. So i did see it with the decks off, and your picture rang an immediate bell. Having said that, there aren't that many ways to make a deck support structure.
My Dad rebuilt it, but I had grown up into motorbikes and girls, so it was given to the local Sea Scouts. They were very grateful, but, as I recall, rigged it and the transom joint split and it got burnt. Didn't tell my Dad though.
Is it carvel or clinker? How long, and d'you see any signs of an aft deck? Mine did have one, but they didn't for much longer.
It was a Proctor MkVI built by Chippendale, and the late John Oakley won the 1956 nationals in it.
It does look like a merlin but not an early proctor. Do you have any more pics. I wrote the definitive design guide back in the early nineties and have a list of all merlins up to then. The number engraved will tell us all.