Green’s Dictionary of Slang

acid drop n.

also acidulated tablet
[the shapes]

a pound, a sovereign.

[UK]‘Sapper’ Black Gang 394: Now if all three had been found adhering to the furniture it might have reconciled you to the loss of those hundred acid drops.
[UK]‘Sapper’ Third Round 538: They offered him a quarter of a million pounds to suppress his discovery. Two hundred and fifty thousand acidulated tablets.