My rad has been recored long ago by a Belfast firm. It is not honeycomb anymore but more of a waffle design. It holds a little over 3 liters (incl the engine block) and that is more than my larger Flying Squirrel honeycomb radiator! See picture:
I will try and flush it. The rusty water was before I cleaned it After that I have done some 400 km and the water now is very clear. But it is still worth a shot. Taking it out is no effort, I have done so several times the last weeks. So I will do the test with the water sometime soon.
To be sure I understand your idea: So take out the rad, hold it upside down (with the fillercap on it of course), pour in let’s say a liter of water, turn it the righ way and see how long it takes to pour out again. Am curious… I did flush my rad vigurously using my garden hose and it was pratically going out one hole as fast as it was going in the other…
But I do like the old school style of this test ;o)