There is no old official cart of NES Donkey Kong that has 50m. It's commonly removed in ports, likely for the same reason that I see so much complaining about it. It's simply the least liked of the levels, and there was only so much (economical) cart space on any platform back then.
There is an unofficial hack which adds a 50m recreation. It's even on Twin Galaxies, for some reason.
There is an official rerelease of the game which adds a 50m recreation. This is called "Donkey Kong Original Edition", and it was released for the Wii and 3DS.
Unfortunately, Original Edition is no longer available for purchase, as both the Wii and 3DS stores have been taken down. Fortunately, Original Edition is an ordinary NES ROM that will play in any NES emulator. I think it's also included in Nintendo Switch Online, but I don't remember. In any event, NES Donkey Kong is a nice version of the game that deserves more respect, and Original Edition deletes the common excuse of "but it doesn't have the piiie leveeel, it's a baaad port".