Back in 2002, back in the dark ages before the waiting lists and hundreds of people clamoring to get in, I went to UBI to try my hand at frame building. Schedule-wise, the class that worked best for me was the steel TIG class. It was a ton of fun, and I learned a lot, but the learning curve on TIG was pretty steep for me - I've often described it as 2 weeks of self-administered shock therapy. It's not easy!
As you might guess, training people to do production welding takes some time, and teaching them to do it in titanium takes even more! Steel is pretty forgiving, but titanium is far, far less tolerant. So naturally, a largish titanium frame production line is going to have their people practice. Apparently, someone at a large Ti frame producer thought this..."air filter" made a perfect project.
As a practice project, it's got it all - mitered joints, tight radius welds, and milled-down tubing. It's owner tells me it has one fatal flaw as an "air filter" - you can't put it down without it tipping over, due to where the holding ring is welded. Bike frame design has been thoroughly refined over the last 100+ years, but every once in a while, someone tries something new that ends up not working as well as the classic triangle frame. Guess the same thing applies to the "air filter".
Any guesses as to what frame manufacturer this came from?