This is a  great  early to mid 1800s gesso mould. Really nicely hand carved, these moulds were used by picture framers and architectural craftsmen. They would press a pliable headed putty mixture, made with glue, resin, linseed oil and whiting/fine chalk/plaster  to get a rapid intricate cast for using on gilt picture frames and furniture.
This mould is a particularly nice example  due to its colour, patina and size.
It measures 16cm h x 11cm w.