This is a -complete- Stanley #50 rabetting plane. I found it brand-new, unused, in the box under a shelf at Murray's Tool Store in Edinburgh, Scotland.
Mr. Murray said it had been there for over 30 years, which has to be true because it was last made in 1961! There's more information on this type of plane here. I got talking about woodworking with Mr. Murray and he said "Have you got a minute?"
He took me into the basement of the shop and showed me his plane collection. He has literally thousands of planes of all types down there, including a couple of 5 foot long coopering planes for making barrels. If you're in Edinburgh and want to see a brilliant shop, just go to 83 Morrison Street, or call +44-131-229-1577.