For many hours that was the build number. I got this on one mac. But many hours later I installed it onto another mac and the build is now 20E5217a. I think that was the intended build number. I think it is a mistake on apple's part - but not a serious one. I think there has been a re-release with the correct build number (unless they are now randomly releasing 2 slightly different simultaneous betas - but that is unlikely).
I think that there is going to be no major difference between 20E217a and 20E5217a and so won't bother trying to "update" the mac with 20E217a to 20E5217a.
However there will probably soon be a full installer. There isn't one yet. There hasn't always been - so don't be too hopeful. Nevertheless apple are getting much better at providing full installers for betas lately.
So going forward the build to look for for the full installer will be 20E5217a, not 20E217a - if there is going to be one.
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