Not Hamer - apparently Burns!
Well, not a 12-string in the modern sense, but still.
A random pic of The Hollies popped up on Facebook the other day, and I was surprised to see Eric Haydock playing what looked like a Burns Double Six instead of his usual Fender VI.
On closer inspection, those strings do look a bit thick... so I did a bit of digging and yes, Burns made a 12-string bass in the 60s. It was even on their now-archived website so potentially was still available as a custom order in recent years - the Double Six Apache model.
It seems that all the featured players of the Apache Double Six played it as a standard 12-string guitar, however the glamour shots on the Burns site are all clearly strung as a bass:
And, it seems it was used for recording as well!
Jump to 1:10 for a slightly hesitant bass solo. Begins well but then he seems to regret starting it at all.