Go to the eCFR - https://www.ecfr.gov/cgi-bin/text-idx?SID=c95f59020a7c0f43313637e3a89639a7&mc=true&node=pt21.4.211&rgn=div5
I strongly recommend the ecfr version. The April date you are seeing on the fda.gov site (or somewhere else) just means the last date that copy was pulled.
You'll see it say Source: 43 FR 45077, Sept. 29, 1978, unless otherwise noted. This is after the contents.
Throughout the document, at end of certain sections, you'll see this sort of language:
[43 FR 45077, Sept. 29, 1978, as amended at 60 FR 4091, Jan. 20, 1995]That tells you when it was last amended and in what federal register.
You can search on the federal register, most likely your local library has access and can show you how.
So basically, 211 is from 1978, except where noted. Mostly those changes are pretty darn negligible.