Previously there was no way to search for 'Type Ins' from the advanced search unless you knew the title. I raised this with @Einar Saukas and he has kindly added a Magazine filter box for us.
This example searches for Spectrum Titles from Sinclair User:
The old way of finding magazine type ins was as follows (but you could not filter the list).
Go to magazines:
https://spectrumcomputing.co.uk/magazines.php
Click on the magazine of choice, and click 'associated titles'