After hours of googling, I think what I have is gel flex, (an identical product apparently is vinamold.) I had this product from years and years ago to make plaster moulds. What I like about it is you can re-melt it, if your mould doesn't work out, so there is no waste.
I melt mine in a jug in the microwave, check on it every 10 seconds, less if it is close to being totally melted as it can quickly go over the temperature and burn.
I have used silicone cupcake moulds to pour the melted gel flex over. It cools down fairly quickly and shows the detail of very delicate things.I hope you might be inspired to have a go at making your own moulds, I will be looking out for old fashioned keys etc to make more.