Looking after my own 51" over strung upright from 1956 and my stepdaughter's 46" stright strung Broadwood from about 1900 the major difference seems to be that the old piano has rusty strings, not that they are brown but patchy black.
I have NO trouble getting good unisons on mine but the Broad- wood is another story completly.
I have a small Yamaha tuning device and if I initially aurally tune the Broadwood's center string for of all the unisons, followed by the other two strings this is what the little box shows - without fail!
The center strings are within about a cent either way but the other two strings are both about 4cents flat.
The above is playable but what I would term "rough".
I may someday go to the expense of new strings just for the experience, but any other observations would be interesting.

I don't really understand the problem. Are you tuning the unisons by ear and the ETD disagrees?

If you confirm the error, can't you correct it with the ETD or by ear?

If your ear disagrees with the ETD, that's not unusual. ETDs do sometimes fall short of a trained ear in certain situations, especially if the ETD is not setup properly. And unisons are often tuned by ear by ETD tuners anyways, for that reason.

Mark thanks for your interest. On the old 1900 straight strung Broadwood and on my 1956 overstrung Brock, I do the following.
1. Mute off all but one string to tune - start from Dsharp3. This is the first on my long bridge on the Brock) up to about D3. I use about the same range on the Broadwood.
2.Set A4 to 440Hz with the "box".
Then by ear only tune this range so that it sounds, to me, acceptable.
3. Wnen 2 is complete I check the single string tuning with the ETD, on both pianos ETD shows that I am taking into account the Railsbeck curve - I.e. Notes are shown as getting sharper by a couple of cents as I progres up the scale, and vica-versa. I suppose this is more by good luck than good judgement!!!
4.With out using the ETD I then tune the other two strings to the "tuned" string.
NOW on the Brock if I take any completed unison and use the ETD to check each string in a unison they will all give the same reading.
On the little Broadood the two outer strings will show up about 4cents flat.
Essentially this would seem to mean that my ear makes a decision that on these old and rusty strings things sound better if the srings are not on the same pitch.
The ETD is made by Yamaha YT250 and has only a scale over 35mm, the needle on center being the desired position and indicated by a green LED.
Just an interesting observation.
On the Broadwood I may some time fit new plain strings, if my stepdaughter allows!!