Mystery maker's mark on vase 1 - brown and tan glaze

I really love this vase. It's not tall - around 15cm high - but it weighs nearly 900g and is reassuringly solid to handle.

The glaze is beautiful and such a rich colour, it feels like it must be made by someone with a lot of skill. However, despite a lot of internet searching, I just can't identify the mark on the base. It could be an ''f'' or perhaps a symbol.

I would love to hear from anyone who recognizes it.