I found this today cleaning up the basement. This is an audio cassette shipped by MITS in 1977 containing an FSK version of Altair (Microsoft) 4K BASIC for the Altair 8800 computer. My Dad and I built our Altair in 1975 and finally got enough memory (5K) to run BASIC.  We used the Altair ACR board hooked up to a cassette tape recorder to load the software. The cassette was also shipped with a snippet of ASR-33 paper proving that this cassette loaded on an ALTAIR- along with a statement of free memory.  I wish I still had the snippet – but it most likely has been lost to the ages. The back side of the cassette has hand written 125/256 so I suspect this was a batch of 256 cassettes. There were apparently earlier versions since this is listed as version 4.0.