Raisin spirit was just one of many possible additives. Contemporary literature refers to fruit extracts (made e.g., from prunes and raisins) which were fruits macerated in neutral spirit with flavourings (e.g. carob). These were added to new or immature spirit to lend it more flavour and cover up (if necessary) off-tastes. Southern Comfort is (in my opinion) a survival of precisely this type of flavouring. The flavoured vodkas and genevers date from this time, so do (I am sure) many of the korns and snapses of Northern Europe. Some American whiskey was flavoured, e.g, Rock and Rye (rye whiskey sweetened with rock - crystal - sugar and citrus fruits), the blended whiskies, etc. Not all that much (really) is new!
Here is Barnard on Laphroig:
"...a thick and pungent spirit of a peculiar 'peat reek' flavour".
We know what he means.
Barnard on Monasterevan Distillery, Co. Kildare, Ireland:
"We tasted some of the "make", six years old, and considered it a fat, creamy whisky".
This could have come out of Murray's 2005 Whisky Bible except Jim wouldn't put make in quotes.
Barnard on John's Lane, Ireland:
"delicious, finer than anything we had hitherto tasted, perfect in flavour, pronounced in the ancient aroma of Irish whisky".
True, somewhat vague, but I think this could have meant the whiskey smelled and tasted like ... Redbreast or Jameson 1780.