I came across this Sylvania F15T8/DS835 Engineering Sample lamp on eBay.  The seller is hyping it as a rare bulb, but besides the etch, it doesn't seem to be anything out of the ordinary.  Do you think this was a lamp produced for internal testing?  If so, it probably is uncommon, but $40 still seems a bit pricey.

those engineering samples, they give those to companys who are designing ballasts or
doing testing on lamps such as burning hours, performance, phosphor samples in the tube. before the lamp get into production is my guess. I have seen them on ebay for a good while. but i am not willing to pay that price for lamps.

I have seen engineering samples in electronics as well, to help companys design
a products around there simiconductor, and test out performaces, before they put them in production lines.

He has multiple samples, different "colors" of white....

funny this seller watermarks and copyrights pic/graphics stolen from donsbulbs.com.

Somewhere I have a CD drive for my old laptop that says "Engineering Sample" on it.

Every year Sylvania issues thousands of Engineering Sample lamps, and many other manufacturers also use the same marking.  They are simply laboratory prototypes which are released for field trials or other joint development work, some months or years before the new lamp is finalised and ready for mass production.  The Eng. Sample or ES etch is necessary to avoid confusion at a later date in case one might accidentally end up in normal service somewhere, or to avoid that somebody might complain if the lamp doesn't give good performance.  Since they are given out free of charge and because the final design specifications might not always be achieved during the early development phases, they are not eligible for any kind of guarantee, and this marking avoids that they might mistakenly be sent back with a complaint in case they don't live up to expectations!