I can’t offer any musical explanation, but it is not a mistake.  The College website confirms the Quintfluit 3 ft and, perhaps more to the point, Flentrop’s website does also: https://www.flentrop.nl/orgel/cambridge-sidney-sussex-college/

I agree.  A Twelfth would be more logical.  Perhaps it is, but just badly named?

A slightly more careful reading of the original poster Peter's contributions than my previous attempt suggests that what he is really looking for is two-manual instruments which have been successfully rebuilt / augmented for congregational / parish work. A firm I do know has done a great deal of 'updating' two manual parish organs in the UK is Henry Groves of Nottingham, often under the advice of Paul Hale whose article is linked above, often within tight budgets and often not fighting shy of extension.  Their 'rebuilds' page is here with, lots of samples of their recent work, and two that caught my eye as being relevant to this thread might be the Quorn  and Newnham rebuilds, among numerous others.  A few caveats: Groves often use non-standard winding architectures, direct electric mechanism and computer controllers to save huge amounts of physical space and achieve flexible and accessible internal layouts in their rebuilds.  These are innovative, but therefore inevitably have not really been through the test of time, and they may not be available in Australia (where I think Peter is based). Many of their 'rebuilds' are almost new organs but with old pipework. I've never played a Groves organ or rebuild, so I have no idea how 'good' they are (perhaps other members have direct experience?). Also I am very aware that some here consider such updating to be mutilation.  Also, while I defer to our kind hosts, their recent work does not seem to be documented very extensively on this site at the moment, so I'm not yet very familiar with their contribution in this area (if anyone does have examples to share then do please step in and fill the gap, as I'm sure it would be of interest).    And other excellent organ builders are of course available! With all that said, Groves's work does appear prima facie to be a constructive match with what Peter was asking to be shown, so I hope the links above help. SC ---- [tl:dr: maybe look into Henry Groves's 2-man rebuilds, but this isn't an advert!]

Yet another - like the previous three on here from me, I've actually played them all and in some cases at services too, so they've stuck in my memory as very fine and serviceable instruments in my estimation.  You will note that they are all by top flight builders (Frobenius, Harrison, Tickell and - this one - Hill, Norman and Beard). St George's, Dunster, Somerset (on the edge of Exmoor) Hill, Norman & Beard, 1962 https://npor.org.uk/survey/N05562 This is somewhat of its time, the mid-20th century.  In a few words I can best describe this organ as perhaps like a two manual version of the Royal Festival Hall organ - maybe trying to be just a bit too eclectic, but no cavils about using electric action, detached console  and a bit of extension and borrowing (though there isn't too much here).  Oh, and a stop key console!  The OP was interested in a beefy sound, and this instrument can certainly provide that when needed, especially with the subs and supers all over the place.  And that's before you give into temptation and use the Trompette en Chamade!  But it also has a wealth of quiet colour available as well. But when I first came across it in 1980 I was pretty much swept away by it and have remained so.  So much so that I simulated it in my virtual pipe organ at home where I've  had years of pleasure from it since. If you fancy a short break down to Exmoor why not pop in and have a look? HN&B at their very best in my view.