Reading through a book of cocktail recipes from the late 1800s, I notice some recipes call specifically for Santa Cruz rum and some call for Jamaican rum. Is this a mere place of origin reference, or is there some qualitative difference? For example, would one of them be what we now call spiced rum? Or light rum vs dark rum?  
Cocktail historian David Wondrich discusses this very topic in his book "Imbibe!", pages 74-75. (It's structured around Jerry Thomas' first cocktail books—perhaps it's in one of those you're seeing the spirit referenced?) The references are indeed to places of origin, but there are qualitative differences as well. Modern designations like "light" and "dark" are essentially meaningless (2, 3). Spiced rum is typically poor-quality, neutral spirit with sugar and artificial flavoring added. It's a style that only took off in the 1980s with the advent of Captain Morgan, and can safely be omitted from any classic (or modern craft) recipe. Rums of the 60s and 70s were characterized by light body and clean taste, in order to compete with vodka, the popular new kid on the block. Their blandness—which opened the doors for Captain Morgan's popularity—was (and in too many cases still is) a far cry from the rums of the 18th, 19th, and early 20th centuries. Jamaican rum was (and still is, in many places) regarded as the "best," or at least the most flavorful. Heavy-bodied, funky, and delicious. Smith & Cross is probably the best easily-sourced rum that might get close to the strength and flavor of rums that one may have seen 150 years ago. Hamilton Pot Still Jamaica and Doctor Bird are also excellent choices. Hampden Estate (which has been in continuous operation since 1753) rum is very good, but current offerings are more expensive than you might want. Rum-Bar and Plantation Xaymaca are fairly solid, 100% pot-still picks, but fairly mild in terms of strength and "hogo." Appleton is solid, though similarly mild. Coruba, Navy Bay, and Myers's are pretty far from the recipe-creator's intent, but worlds better than, say, Bacardi. Santa Cruz rum—the "un-Jamaican,"—was lighter than the Jamaican rums of the era, but not nearly so light as today's multicolumn rums from St. Croix, Cuba, or Puerto Rico. Given how little we know about the Cruzan rums of yesteryear, you've got a fair bit of leeway. Try to find a rum that's got at least some age (which shouldn't be guessed by the color), some backbone and bite but not too funky. A tough of vanilla from the oak and grass from the cane. Wondrich offers the following blend to approximate the profile of Sta. Cruz rum: 1 part Smith & Cross (or other full-proof, 100% pot-still Jamaican rum), 2 parts lightly aged rhum agricole (paille or ESB), 3-4 parts young Cuban-style rum (like Don Q or Havana Club). To keep things simple, you can probably get "close enough" with a milder blended rum like Appleton Signature, Denizen Merchant's Reserve or Dark Vatted, Chairman's Reserve, Mount Gay Black Barrel (Eclipse is OK but pretty rough with a too-big hit of ethanol), the Probitas/Veritas collaboration from Foursquare and Hampden, a mild-ish pot-still rum like Rum-Bar Gold, or even something US-domestic like Richland or a local rum distillery you want to support. If you have a favorite rum (not a spiced/flavored one), try it in a recipe and see what you think! Heck, even Cruzan—as different today to its forebears as it is—should still be OK. Also, given this was originally posted nearly 8 years ago, let us know if you have any updates :) Cheers! 