I bought a case on eBay some time back . The guy still has some, but they AREN'T Vitalite.  I have 2 older real Duro Test Vitalites and they look decidedly cooler in temperature than the Sylvania made ones. I did not have a Design 50, but I did have a 15 watt GE Chroma 50 and they matched almost perfectly, but not EXACTLY.  The Phillips Natural Sunshine 15 watter is cooler than the Sylvania Vitalite but warmer than the real Duro Test. I am wondering if it really is a Design 50 or the old Optima 50 formula.  The date code top to bottom is E9VI.  They are typcially late Sylvania, being super swirlers when first fired up and would NOT start in desk lamps that were push to start. They would start on preheat with a lot of flashing, or trigger starting with tons of swirling and shimmering. Would I buy a case again if I needed them ?  Yes.....   

i got a case coming along the way i figure it not the real deal it have duro-test on the box as well
but since i dont have any vita-lite in my collection so i figure least somewhere to start. and it will be
my first see the 5k in action.

Those Design 50's are really nice looking lamps, but they really don't like the cold as I have found out.

posted this earlier but somehow it did not stick.
i got some from the same lot e9v1.so 1999?i owned mine before 09 so before then .
yours must have come from the same guy as i have had mine several years.
they are fussy starting for the first time till broke in.
i suspect they are merely /950.they match some sylvania f17t8/950 i have here perfectly.
despite the fancy stuff in the patent i suspect these have always been a standard /950,c50,ect.
i have seen them from several mfrs.i have some that look like ge's.some like sylvania.

Vitalite is 5500K. I need to light them up next to my 20 watt Vitalite and see.  I remember the Optima 50 and VitaLite were visually different.  I think these lamps were made after DuroTest went bankrupt and someone got the name and tubes were no longer made in house. I wonder where their equipment went. SOME of their lamps really lasted a LONG time.

Powell

I got mine in. i pop on in my preheater i acted like EOL lamp. took 15 min of EOL flashing before it strike the first arc on the tube. but it snake like crazy as well. i like the color temprature and i was lot better than cool whites. so this is my first tube.