Richard, I found absolutely nothing on the net (except my own posts on TM from 2018). There is no reference to tyre dimensions in any Morgan brochure. I know that my 2010 Roadster Sport new from the factory had 205/65/15 tires on black painted wire wheels. It rode very well and it gave more ground clearance. So far I have not seen anyone have 65 tires on the nice alloy wheels but it would be worth a try. It might look good on that nice black roadster of Nestor.

Here you can see the 2010 Roadster with her arc filling tyres.

https://www.auto-brochures.com/makes/morgan/morgan_int-classic_2010.pdf

The only other model that had 205/65/15 was the Plus4SS with her Minilite wheels. I have no idea if only the Roadster Sports were fitted with 65 tyres or if other S3 Roadsters had followed as well or if MMC went back to 60 tyres.

'14 4/4 graphite grey