The HD-266A is a flange tube. Similar to the 15GP22 and others, this tube had its high voltage applied to a metal flange that isolated the faceplate from the funnel. Its screen area of 260 square inches was only slightly more than the round RCA 21AXP22 of the same era.
The Set: Pete Deksnis's Site about the CT-100
Restoring a Vintage Color Television Set