Series 3 seat base

I have recently finished a late series 3 rebuild with adjustable seat runners and seat bases. I am currently coming to the end of a complete refurb of a 1973 series 3 with non adjustable seats. There were no seat bases and I would like to know if this is correct and if so what secures the seat squab.

On the s2 there are little tabs that stop the cushions from sliding forward

Are they plastic a described as cushion corners? Thanks for the response

On my s2 they are metal. I doubt plastic would be strong enough

Thanks