Not sure I agree with the first statement. According to their website, Dallmeier developed the world's first video sensor with digital picture memory in 1992. I posit that device only recorded still frames and therefore was not a true DVR. The first actual Digital Video Recorder would probably be ReplayTV, introduced at the 1999 CES, the same time as the first TIVO. CCTV DVRs are actually an adaptation of that technology, just like many other CCTV products, including the cameras and recorders themselves, which have their have roots in the Consumer Electronics field.
Dallmeier is about as closed an architecture as you can find. Their cameras are not supported by any other VMS, as far as I know, nor do they support other manufacturers' IP devices, despite their claims to the contrary. RTSP stream support is not true IP camera support.
Dallmeier knows that the world is passing them by, something I'm proud to say I had a hand in. Nevertheless, it will take them time to modernize, especially since their CEO is apparently resistant to opening up their platform. Although I don't agree with it, I can understand his viewpoint.
I have no idea who was the first manufacturer to release a DVR though I am not sure it would have any impact on product selection today.
Dallmeier does list many of their IP cameras as ONVIF compliant (but, importantly, not Profile S). Check to see if your camera models are listed there. If so, you can try it out with your preferred open source system. Also, ask Dallmeier what version of ONVIF they are using (according to the ONVIF site, it's an older one, 1.x).
As Carl said, and based on his recent evaluation of them, set your expectations low here :)
Many Thanks to both of you for your valuable suggestions.. Let me check for some of the alternatives.