As for the tuning of modern flutes, assume nothing - it all depends on whether they are intended as accurate copies of originals or “modified for modern use”, even if fully keyed. Also, the compromises involved in even keyless flutes often lead to the open C# and the F# being flat anyway (see recent thread about this feature of Seery’s flutes)- which is why there is the need to vent the keys on the old ones - they weren’t made that way for the hell of it…
One signal advantage the modern flute maker for ITM has over his C19th forebears is that he need only build flutes to be in tune at A=440 with some scope to tune a little either side of that according to environmental circumstances/influences and the occasional tuned a bit sharp or flat context. Another is that he need not (unless he wants to) worry so much about 3rd 8ve response and performance. His scale length and tone-hole distribution and sizing can therefore be better aimed at that specific pitch and the bottom and middle octaves, reducing but not eliminating the conflicting issues. A mid C19th flute maker had to try to allow for tolerably lippable tuning at quite a wide range of pitches from maybe A=430 up to A=455, and where he centred his compromises will be significant to the flute’s friendliness to use at 440 (without it necessarily being a specifically HP instrument). Terry McGee explores all this much better than I can summarise it in various parts of his website. All that said, the modern maker still has to bang his head against the issues of what an average hand can comfortably cope with and other venting and tuning problems in placing and sizing the six open holes.
I would be very leary of tampering unalterably with a top end, highly desirable antique flute - you might well jigger its value even if you successfully “improve” its intonation without obviously damaging it or screwing it up. Anonymous or even named but not majorly desirable mid range ones, OK, go for it, but not R&Rs, Boosey Prattens, etc. You can safely deal with the sharp A by applying a little beeswax in the A tone-hole to flatten it - quite durable and adjustable and totally reversible without damage (I have done this to my R&R). But don’t go enlarging any toneholes - that is effectively irreversible and not so sure a cure as you might imagine, bearing with it the risk of upsetting other notes. Don’t damage your flute. Better to accustom yourself to venting the keys when you have any sustained or prominent F#s or C#s. I do it. Disciplined practise for a few weeks, especially on new-learn tunes, will soon crack it. After all, you have one of the top-end tools - just learn to use it for what it is! Quite a few good modern maker keyed flutes that I have played have still needed venting if one is being finnicky about intonation, and it is one of the reasons I am not a fan of keyless flutes which lack the facility to vent when they often actually would benefit from it! All that said, many of the greats of ITM don’t bother with venting and don’t seem to need it even when playing antiques with, by modern standards, dubious tuning. Unless the flatness is gross, it doesn’t really show in fast passing notes.
All that said, if you have one of the flutes that was definitely built centred on cA=450, the scale length will be rather too short ever to play happily at 440 and you will find you can pull the head well out and either tune the L hand tolerably but have the R hand sharp, or have the R hand in tune but the L hand flat, both of which show badly across the break. Adding a longer head will reduce the amount of tuning slide you have to open, but won’t change the distance between the embouchure hole and the tone-holes at whatever pitch you chose to tune to… modification of the scale by replacing the body with one scaled for modern use (rather than the head) would be more logical, or re-reaming the bore and enlarging some of the tone-holes or even stopping them up and redrilling them would be necessary - not a DIY job, and again, I’d say, not the thing to contemplate doing to a good antique. Better to leave it be and play something else if you can’t modify it sufficiently non-invasively and adapt yourself to using it as it was designed to be used.
