Hey,
I've got a steel Denti frame from the early 80's, I think, and a lot of searching has turned up not much information.
Mino Denti was a time trialist turned frame builder. Denti mainly seems to be known for its track bikes with curved seat tubes. From what I can tell, people seem to like them. He's apparently not involved in cycling much now, but the company still seems to exist:
[search "dentibiciclette" - sorry, can't post links yet]
Matrix tubing seems to have been a short lived columbus line, looks like they had to drop the name because of trademark issues with another company. A bit more detail here:
[search "School me on Columbus tubing, specifically Columbus SL]
I hope that helps, I'm pretty new at this so probably others can tell you something more useful.
John