What an unusual piece! That's surely one of the many joys of studio glass - finding one-off oddities such as this. In this part of our world, you can virtually never say 'never'....I have to say that I've not yet seen one like this, and the following is said without handling it....
- I don't think it's IOW - in general colours were applied to a colourless base there, rather than being melted in the pot as at Mdina. The colours here also seem more correct for Mdina, particularly the addition of the silver chloride and its effects on the base colour in this way. So, I do think it's Mdina - remember that trails could be applied to Side Stripe vases, and 'wings' to Fish vases, and also that general levels of skill rose considerably when the Boffos joined. They made jugs and other tableware at Whitefriars before that, and before that worked in Italy, so could easily have added a pad base. I'd think that, by then, Michael Harris himself would also have had the skill to add one as well.
- Feeling and holding it will help enormously, but the 'chunkiness', for want of a better word indicates Mdina.
- However, as everyone has said, it is very unusual to have a base like that, especially with a broken pontil mark. It may have been made before the company bought its own grinding machine, and IF it's an experimental piece from sometime after or around late 1969 as I think it probably is, maybe they didn't feel it was worth having it ground flat. Or maybe someone bought it 'off the shelves', so to speak, when they visited the factory and before it could be sent off.
We'll never know...but in short, I think it's an experimental or one-off form (there is a difference) from late 1969-70, and may have been made by Michael Harris and/or one of the Boffos.
Hope this helps,
Mark