Unknown piston filler, probably from Germany

I'm seeking help from any knowledgeable pen friend with identification of a nice German piston filler of unknown make that I recently acquired from German eBay.

The pen is devoid of any identification mark. The nib is marked 'warranted' '585 14k'. There is an ink window, but is thoroughly blackened by old ink. The feed is a flat one. The nib too is almost flat, with the shoulders that curve over the feed. By the way, it flexes with little to no pressure and has a good snap back. It flexes from medium to double broad only, though.

The only thing that I'm quite certain of is that this pen is a high quality one, as evidenced from the very good fit and finish. It's also quite big especially considering the fact that it's probably vintage, given its flat feed and flexible nib. It's marginally bigger than a P51.

Any help?

Nice pen!

Since the high build quality, id say probably 2nd tier, but most likely 3rd tier since it has no name on the clip and its a Warranted nibed pen. To clean out the pen, get some luke warm water, and just keep flushing (10ish times). After that, put a few drops of soap in, and flush a few more times, and than once more with clean water. That was you can see the ink window.