I work at an Electra dealer and know the rack and bike you're talking about. The problem lies with the construction of the cruiser frame. There isn't a good place to put the braze-ons for the rack at the seatstay junction. Modern ones are actually threaded rivets that are riveted into a drilled hole, rather than brazed into the frame with solder. This tends to be a weaker joint and as the tubing gets smaller, it becomes a weak point with a hole that sized drilled into it. Look at how big the seatstay is on most modern hybrids where the rack mounts are vs. your cruiser.

While the replacement seatpost clamp with rack mounts is the perfect solution, it really isn't something that makes sense to put on a stock bike (costs and aesthetics) when the majority of folks buying the bike will not buy a rack. At first thought, it would make sense to sell the rack with the included adapter, but seat post sizes have varied, meaning the seatpost clamp is not a one-size-fits-all deal.

I will say this: Electra, for all they do to provide as many accessories as possible, have next to nothing in terms of product description on their website for accessories. I was actually looking at some fenders of theirs for a personal bike of mine, and had to do a google image search to figure out what hardware it did or didn't come with.

My best recommendation would be to work with the dealer you bought the bike from. If it's a good Electra shop, they tend to keep good stock on accessories. I've outfitted many, many of their bikes and am familiar with their quirks and needs. Talking to a knowledgeable dealer will save you loads of time in narrowing down what you want for the bike and what does and doesn't work. And if they're not a-holes, they'll generally install things at no cost if you bought the bike from them.

EDIT: They do offer the adapter BTW: http://store.electrabike.com/eSource...V2.aspx?store=