I see, I guess the code on mine may be upside down and the year code could be 89? Or we're frames all 1" steerers back then? Mine looks to be 1.1/8" which is good as I have no 1" steerer tube forks handy.
As for the searpost, for me it gets hot for around 30 minutes then it goes cooler after that, even tho the mixture is still fizzing and bubbling a little. I'm dissolving a lot of seat post however, I've got around 1ft of seat post that needs melting.
I think I need to drain what I have in there and put some fresh in, that video is good also, the camera guy didn't expect those fumes
Anyway, I think I may have a go at adding some disc mount myself, a friend has offered to lend me their welder and being a steel frame things like this are easier to DIY.