I have purchased Boehm flutes from the London flute ebay sellers, and spoken with him on the phone, he is a pro orchestral player and really does know his stuff, i'm pretty sure if his info on the HP model he has for sale was incorrect, he would correct it very quickly as he has a very good reputation as a seller, and being a pro player as well i would be surprised if he didn't know a lot about tuning on new and old flutes.
While accepting your personal experience and the spirit of what you write, Yvie, and that it may well be true in general of this seller, I'm afraid that while he might believe it, it really cannot
be true that a flute with the specified sounding length will play at CP with internal scale intonation acceptable even to amateur standards. The physics of the matter are immutable, as can be the need to redeem the value one may have in an item one has lavished a good deal of specialist work on and is trying to sell. That so desirable thing (in limited circles, but they are a rare commodity) as a CP-playable and ready-to-play Radcliff has not been snapped up in several months on offer and at least 3 eBay re-listings at what would be a pretty reasonable price for a good CP one speaks volumes. Those in the know...... well, they know.
To clarify, to get a low C in tune for CP, the sounding length needs to be in the range 598-603mm. This flute's SL is 576mm according to the vendor. That would require a slide extension of c24mm - almost an inch. Rudall Carte flutes of this type/era have an upper tenon c 27-30mm long. The maximum feasibly playable slide extension (and with the head wobbly!) is 20mm - insufficient to get the C down to CP. But even if the C could be got down to CP, then the whole rest of the scale would be flat - progressively more so going up to the top of each octave. Extension to CP tune the A would be about the maximum truly secure position, leaving no real scope for further flattening in hot & humid conditions. Extend the slide to get the A (or any other given note) in tune at CP and the notes below the reference note will be sharp and those above it will be flat, more noticeably so the further away from the reference note.
It may be feasible for slow demonstration purposes to lip individual notes in and one's way through a scale, but the embouchure gymnastics required, even with much practice, preclude this being a satisfactory playing strategy. Like I said, I've been there of old with an HP Bohm system which had had the upper body extended, and I have both CP and HP RC&Co flutes currently. The doctored Bohm one played beautifully in terms of tone and response, but due to the extension the L hand notes were flat and the foot end sharp. It got very frustrating. I have a lovely HP 1867 system which I can, in like fashion, coax and push to kinda play at CP with the kind of head extension our Radcliff seller is suggesting and with a very downward, flat-blowing embouchure approach; but start trying to play along with a recording or a fixed pitch instrument like a piano and the discrepancies show, quite obviously, even to my not especially discerning ear.