Rear seat LSTC

[airtids]

Looking at fitting a sixth seat into our 1965 337. It came without the rear seat next to the baggage door (emergency exit?), and has no rails for that position. Does anyone know what the requirement is for an LSTC (Limited Supplemental Type Certificate, or sometimes called a 'one-off'). It seems to me that the aircraft is certified for six seats, you should be able to put six seats in it without any need for approval. Does anyone have any information on this, and suggestions as to sourcing seat rails for that position? Thanks.