AC only, the Delco part of the brand name was dropped years ago, as far as spark plugs. And AC spark plugs are no longer being produced. Here's a possible source: Link
Spark plugs are pretty generic nowadays, as long as they're the proper configuration, any brand should work fine, IMO.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.