So I had a bit of a ponder during a long drive, and did some more reading. I do hope that self-answering doesn't have a coherency/rambling ratio threshold…
Anyway, the Marshalsea records are a substantial, if rather unwieldy, data set. So maybe it can yield more than it already has.
First, the committal record. I couldn't find clear, contemporary definitions of "damages" and "sums sworn". My first thought was that "sums sworn" is the amount outstanding, while "damages" is an additional penalty. As an alternative, perhaps "damages" may then have meant the amount owed, in which case "sums sworn" could be an amount the debtor had to pay to leave prison. That seems more realistic, not least because those are sums the prison might need to know, rather than a breakdown of total debt.
So what can the data tell us? We should find for case #1:
- Damages and Sums Sworn need not correlate closely in value (penalties may have contexts)
- Total to pay = Damages + Sums Sworn
And for case #2:
- Sums Sworn must be less than or equal to Damages
- Total to pay = Sums Sworn
A survey of many pages indicates that Sums Sworn can be equal to Damages, or lower, but does not appear higher. Equal or about half are the most common values. That is consistent with case #2. So Sums Sworn forms the "get out of jail expensively" card.
The bit after the Sums Sworn presumably isn't "cupboards", although it does look like it begins with "c". However, this blog references Marshalsea records, and states it as "upwards" (so "upwds" as written). Which makes sense - pay at least this much to get out. The initial "u" must just have an excessive lead-in scroll.
So for Demetrius, it looks like his debt was £40, and he needed to repay upwards of £20 to leave Marshalsea. Given his apparent transfer to Kings Bench, I guess he didn't repay it all during his 6 week stay. (I've looked through Ancestry's KB records for 1812 and 1813, though, and he's not there under "R" or "D".)
Moving to the discharge record:
It occurred to me while reading around that "P.F." might be Prison Fees. If so, I expected it to scale with incarceration length. But it doesn't. In every example I've looked at, it's "10/10". Still, this 1815 report into Kings Bench and Marshalsea notes that in the latter, "Fees were, however, exacted to the amount of 10s. 10d." So that fits, and suggests that the other (CC, CM) columns, where monetary, are shillings/pence rather than pounds/shillings, and may also be some sort of fees. They are not always completed, while PF usually is. None of them bear any relation to "sums sworn", so they don't appear to reflect repayment.
The "R.R." column is, in later books, "By whom received", and is normally initialled. So the clerk, magistrate etc accepting the fees. An example is for Charles Dickens' father John, highlighted in this report on the release of the records.
I'll update this if/when I get more information.
Update 11 Aug:
I have found a paper on debtors prisons from the Australian Journal of Law and Society. Although I haven't fully digested it yet, it does note on page number 64:
Wealthier debtors had themselves transferred by habeas corpus to the Fleet or King’s Bench prisons, where pleasant conditions inside or even outside the prison walls (in the prisons’ “Rules”) could be bought. In these prisons, in particular, debtors ran their own lives and could indulge in drinking and debauchery if they wished.
A habeas corpus writ essentially asserts wrongful imprisonment. While it might not result in release, it could clearly effect a transfer to a less unhospitable gaol.
So "HC to KB" could well mean "Habeas Corpus to King's Bench". I don't know if Demetrius was "wealthy" or not at the time, but this does suggest some resourcefulness and knowledge of the law, which is interesting in itself.
In addition, I've had another look at the King's Bench 1812 and 1813 prisoner lists on Ancestry. Demetrius appears in neither, as far as I can see. It's not clear from the books or supporting information when they were drawn up. This needs a more thorough study to be sure, but I think that the 1812 list was constructed some time before the end of the year in response to new laws, so may have missed Demetrius' transfer in October. His absence from the 1813 list then suggests that he was discharged from King's Bench fairly quickly, probably before the end of 1812.