M-21 separation speed

Like me, most of you might have read a helluvalot about A-12 / SR-71 / YF-12A / Blackbird / and mainly M-21. For those who didn’t, Wiki is not bad, see http://en.wikipedia.org/wiki/Lockheed_M-21
But what I could not find was reliant information about at which speed exactly the separation between M-21 and D-21 occurred. Any trustworthy source, anybody?