Well, two things I envy you for: this nice
Galba, and the fact that you can go to the BM in your lunch break. Great!
I can't tell the colour so clearly; could it be not an
orichalcum dupondius but a bronze as? A such is described in
RIC, no. 295,
rev. LIBERTAS-AUGUS, S-C in
field. 294 would be LIBERT-AUG which obviously isn't your specimen. The
obverse (laureate
head r.,
SER GALBA IMP CAES AUG TRP) would fit. It's rated
Rare, but you must consider that
RIC only uses C, S, and R in this series (no R2, R3 and so on), so R really means
rare.
Best regards,
Rupert