Perhaps you should consider looking at
https://www.dellowregister.co.uk/ to find the history and numerous pictures.
Listerjp2 has come up with his interpretation (Mk2) it’s interesting, but missing the extra detent to select reverse, and works for a Marina based Marlin Roadster and may well work with Triumph based Roadsters which have the same sort of mechanical assembly and history.
Dellow have their own history, much older than Marlins, of course they get customised dashboards, but Marlins were always a kit car, so subject to personal interpretation!