Assume nothing. There were two plume marked 2a halfcrowns in lot 268 (bought by Crowther), one VF and the other nearly so but small. Both had the number of pellets as stops listed, and it doesn't match. We can be reasonably confident the coin was Burstal's despite this inconsistency as the ticket has FOL plus the number and that matches FOL 112 on the two tickets imaged in Eaglen's article. An explanation of what FOL means would be useful with the number change. It's worth noting that one of the tickets in the BNJ with FOL 112 on it was acquired from Seaby in 1948, so this may have been acquired prior to that date.
Other disposals noted in Manville & Robertson were made in 1957, but that was milled silver, and 1984 was hammered gold. I don't have old man Burstal's catalogue, so can't check that.
On somewhat more solid ground. Hawkins 2a and Francis 2a with mm. plume are the same. Not in Francis is correct. None of the 17 obverse readings used HIBE. The 5 after the lower 2a must refer to the Francis harp type as it is the only flat fronted harp he listed. He doesn't list a plume marked reverse with 5 pellets left/1 pellet right of mark, but notes 2 reverses for the following mark rose. The first has a different harp to that seen, but significantly this harp is used on the second rose marked reverse (Francis 5), with the added bonus of Francis noting that it has a pellet each side of and between C R. I think we can eliminate the 5 as referring to the reverse number as the mark is wrong. It is therefore worth checking any rose marked examples with this pellet feature to see if this is the same die recut with the new mark. If it is, it would probably place the die right at the end of plume. Brooker 301 uses the same harp, but is a different reverse die. He didn't have an obverse reading HIBE either.
S2113 places that prior to 1977 when the Seaby references changed.
Thinking out loud, I wonder if he passed a number of coins to offspring, as the number of Tower halfcrowns in the '68 sale was little more than a type collection exercise.