I just bagged this one off the e-place. Large green Folembray 728 stand-off insulator.

4 1/2 inch tall, 2 1/2 inch diameter. Threaded at both ends, with solid glass in the center.

This is indeed a 728 and not a 738. I'll post another photo of the embossing.

Thanks to Lis Bergman for pointing this out to me! Lis also informed me that the Folembray spools were made in various versions, including 718, 722, 724, 725, 727, 728 and 733. These are also referred to as "accordion" insulators. This one is most likely a CD 1061