Yes, unlimited! 
but only in the new singles category! full story here -
https://manxgrandprix.co.uk/latest/page/...ghtweight/

Obviously a good move to re-introduce what was basically the ultra-lightweight class, as the entry size has basically doubled. I was hoping someone quick would have a go on one of the 690 Kramers and Shaun Anderson is listed as riding one. Really looking forward to this race now. Roll on the 18th!

Yep, should be good, it's going to be a whole new ball game.....inside the original ball game!!
A bit like the Lightweight, UltraLightweight & Singles TT all rolled into 1! 

Interestingly, about tyres, regs state - 250cc all single cylinder 4 stroke machines ridden in the Lightweight Race may use slick tyres. 
Other classes may only use tyres having a moulded tread pattern

presumably they've missed an '&' out?

I would think so. I would imagine it's because the 250s and singles are all pure race bikes but the 400s and Junior Supersports are all derived from road bikes....probably!