Ilford Multigrade split-grade exposure calculator how was it actually used?

I recently came across this Ilford Multigrade exposure calculator disc (photo attached).

It appears to be an older, pre-electronic darkroom tool related to split-grade printing, but I haven’t been able to find a clear explanation of its intended, practical use.

I’m hoping someone here may have:
used this disc back in the day,
seen original Ilford documentation,
or can explain how it was meant to be read at the enlarger.

In particular, I’m trying to understand:

what the inner numerical ring represents,
how it is meant to be read against the outer time scale once the disc is set,
and how the blue-labelled filter combinations (e.g. 0 + 4, 2½ + 3, etc.) translate into actual exposure steps in the darkroom.

I’d really like to understand how Ilford expected a printer to use this tool in practice, step by step.

Thanks in advance.

It has no relationship to split grade, but is intended to allow you to read off relative exposures for an older set of Ilford Multigrade filters before the current constant exposure ones came to the market.

There are however relationships that can be established in terms of exposure differentials in stops between 00 & 5 filters that reveal the actual grade in use. Or you can use a densitometer and avoid all that.