What is the Q506 on a TurboExpress?

[pipi]

Hi to everyone,

I summon you, pc engine lovers.

A great Modder, Leon Kiriliuk, needs some help.
I thought that if the information is not here well..
It doesn't exist

Does anyone know what Q506 is on a Turbo Express?

Tweet:
https://twitter.com/leonkiriliuk/status/1631884219495469056?t=_eUd4o3k_b4mj8cKJjkYgQ&s=19

Thanks to you all!