The FBM body is used as a buck, and RTV silicone is used for the mould itself. Once hardened, the mould retains the shape of the FBM body, so you can pour with soft plastic lure liquid of whatever hard/softness you choose to make bodies for pennies. I use the lure liquid from Lure Factors for my bodies, in Medium or Soft grade.
Here are some I poured from the FBM mould I made. Shop bought lure at the top. Mine below.
And showing the belly slits for the hooks.
@Maverick, hope it's ok to cross-link to one of our OO articles?
https://www.offshoreoutlaws.co.uk/articles.html/articles/how-to-design-and-make-soft-plastic-lure-moulds-r6/
@alun j., the FBM mould was only a one part top pour mould, rather than the two part mould in the article but the basic principles are the same. I don't have the mould with me or I'd share some pics of that too.