Pongráctelep is a really nice residential area in Kőbánya. It was built between 1939–1941 and has great historical background.
Here's a fulfilling article about it in Hungarian:
https://hu.wikipedia.org/wiki/Pongr%C3%A1ctelep
The cache is hidden next to one of the 3 remaining original murals, so you can enjoy this historical work of art (socoalist, though)
There's a logbook and a pencill inside.
Be discreet, please, the place can be crowded.