Catalogue of Coins in the British Museum - Vandals, Ostrogoths and Lombards - Warwick Wroth (1911).pdf https://commons.wikimedia.org/w/ind...nd_Lombards_-_Warwick_Wroth_(1911).pdf&page=5
I thought the pound back then was nearly $3 which would make the 12/6 Gordian more like $4.35. As I recall. you could get such a coin for $5 when I was in college but that seemed like a lot back then when I felt lucky to be making $1 an hour as a part time nobody.
I think it had that value around WW I, & after WW II it took a steep fall close to where it is now around $1.37.
I just did a Google search & you're very close . In 1966 a GBP was worth $2.80, & in 1967 it was revalued at $2.40. Today it's worth $1.37. That should make the antoninianus worth $1.75 in 1966, calculating 20 shillings to a GBP.