Early versions of the Durotest krypton bulbs had this weird shape. The bulbs were made in '74, but I have a couple from around '70 and one from '77. Later versions of Durotest krypton bulbs came in the AT19 shape they were like regular bulbs but with BT style ends on tops. The bulbs shown here are kinda bright for a long life incandescent!
Maybe! KT-17 sounds technically correct. Yep the smaller envelope was used for krypton fill but it seems those made in the 80s and after came with regular sizes.
Here is my picture of an european K shaped krypton filled incandescent lamp (GE Hungary 60W lamp).
It looks significantly different than this lamp (Indeed, for this lamp, AT-17 is better name than KT-17, since it didn't look like a mushroom at all).
The A shape is the standard shape for the GLS bulbs (A for "Arbitrary"). In my opinion its shape is much closer to the K shape than A shape. And the T added to some bulbs designates the tubular bulge present on the top of them.
The more I think about it, I think the Durotest bulbs above most closely resembles a M shape. Some of you might remember GE briefly marketed a fan bulb in the MB-14 shape back in the early 2000s. So the bulbs here might be MT-17s!
This krypton incandescent lamp, used in this case as an energy saver, to produce the same amount of light at less energy instead of producing 10% more light from the same wattage as in most european krypton incandescent lamps.
Both techniques gives longer life.
Duro-Test later made a full wattage product called the ATM-9 Superlife. They were rated at 7000 hours, twice the life of Duro-Test's 3500 hour bulbs. Thanks to the krypton fill gas, you get zero lumen loss for double the life. A 100w ATM-9 is 1300 lumens vs the 3500 hour's 1290 lumens.
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It looks significantly different than this lamp (Indeed, for this lamp, AT-17 is better name than KT-17, since it didn't look like a mushroom at all).
It's all described right here .
Both techniques gives longer life.