So there is an auction schedued for the fourth of May and the seller is advertising an Audemars Piguet with a 'Beta 21' movement with an estimated price of €8,000. Thing is, it isn't a Beta 21 movement at all. It is a rebranded Omega 1510 or 1511 movement. This is (essentially) the same 2.4 MHz movement used in Omega's 1974 Marine Chronometer.
I have e-mailed the seller to correct their listing but they probably won't take any notice. No-one else ever does.
Still, if anyone is interested in old MHz-range HAQs and was interested in something just a little more unusual than the ubiquitous Omega MC (and if they have deep pockets and more money than sense) then keep an eye on this auction!