I've Googled my fingers off, but can't manage to find to which of the many Tosos the signature refers! I've found a few examples of the signature, including one in Christies' auction archive, and several of them estimate 1950s production - but it feels more 1970s to me (but I might be wrong).
The vase is 30cm / 12" tall, weighs an absolute ton, and has a ground base. The signature is on the inside top rim of the vase.
Thanks in advance for any suggestions.