Have just got this one. It's firmly fixed into the base of a well patina'd No. 5.
I've not seen this one before,
The maker looks like L T H Ltd but it could be T H Ltd. If that's the case it's Thomas Holt of Rochdale, if not I don't know who L T H Ltd are. It's a different plug to the Thomas Holt on Dave's website.
The plug has a central bar. The photo is deceptive as it looks like a groove.
Suggestions welcomed.

I've not seen this one before,
The maker looks like L T H Ltd but it could be T H Ltd. If that's the case it's Thomas Holt of Rochdale, if not I don't know who L T H Ltd are. It's a different plug to the Thomas Holt on Dave's website.
The plug has a central bar. The photo is deceptive as it looks like a groove.
Suggestions welcomed.
