Should be perfectly drinkable! (You might want to after this...)
You bottle markings "No A1" is probably an "AI", but the "C61458" eludes
me as I have no references to a serial nuber in White Horse that uses a
Letter as the first character.
The "36-063430" is from a US Tax Stamp, and is usually found on older
bottles of spirits dating to around the late 1930's, as would a White Horse
serial number in the AI 6-digit range...If this was from the late 1930's era,
it would be worth about $500-$800 depending on it's general condition.
Great Scotch though...much better than the bottles produced from 1960+.
Walter C Hurst