I did some research on your Dad's rod and am 99% sure it was built by Horrocks-Ibbotson.

You are correct in that it's not a valuable rod but it was built on a good blank. If it has no hookkeeper and has a snake type stripping guide, with minimal line guides, you can be sure it is a H-I.

No luck on the line weight but I would try a 6 first.

I can certainly understand the rods value to you.
I currently own all of my Dad's tackle as he can longer handle it....he'll be 94 next month.
It holds great sentimental value for me too!

Bob