Filter size for Zeiss Ikonta 521

[Minox]

Hi all. I'd like to get a yellow filter and a lens hood for my Zeiss Ikonta 521 camera; it's the 6x4.5 version, the 1/500 Opton Tessar lens. As far as I can tell, this would be a snap-on, as I do nto see any filter threads on the lens.

Anyone ? Thank you !