that's not true at all several sources state that A-12's and SR-71's were declassified at roughly the same time. also they were in the rumor mill long before they were publicly revealed.
sorry, thats rubbish
the RS/SR-71's did not appear at the same time as one was a legacy of the other because of a mission change.
the rumour mill also trotted out the A-19 - when the F-117 was rolled out 5 years after being in operation it looked nothing like what all the idiots on the internet were claiming
anyone who claims that the RS/SR-71 were fielded at a development level at the same time is talking absolute errant nonsense.
the significant developments made in LO profiling were done with the RS/SR-71. The A-12 never had the fitout - as it was the platform changes made to the habu were accidental. they were deliberately designed in but even johnson and rich never realised the benefits until months later..
as for declassification, I can tell you point blank that when I asked for the release of some material less than two months ago, it came back very clearly that fitout and details of the sensor suites were still privileged material
all comms gear on display at various museums, even today, are gutted and some sensor gear is physically removed. This includes gear dated from the late 50's