@Edlothiad and @Cherubel:
•Pre-War of the Beast (but post-Iron Cage) IF is set up with 50 Companies, or Walls: Note that the "wall-brothers" are the Company Champions of each Wall/Company, charged to stand guard atop each Company's namesake wall at the Imperial Palace.
I am Slaughter, ch.15
‘It would be the worst loss imaginable,’ agreed Zarathustra. ‘For the Imperial Fists, the Old Seventh, greatest of all the Adeptus Astartes, and the most loyal of all defenders of Terra, to be reduced to… to nothing, to nothing but the last fifty wall-brothers stationed at the Palace. To lose all but five per cent, to be diminished to a twentieth… How would we ever recover from that?’
• That thing which happened in Book 10:
The Last Son of Dorn, ch18
The prime-ork had known exactly what had to be destroyed. To eliminate the one thing its rival could not afford to lose.
‘I think… I think that…’
‘No!’ said Bohemond. ‘No, I will not accept it. The line of Dorn must live on.’
They had lost. The last son of Dorn was dead.
Shadow of Ullanor, ch3
‘We all know how it wounded Dorn to break his Legion. The grief of it never left him. Imagine how he would have felt to see this day come to pass. Imagine what he would do now, if he could have attended this Feast. What he would tell us. Of course, the loss of the Imperial Fists will be remembered – will be honoured by all the Chapters here present. But all of the Chapters here were Imperial Fists once… and could be again.’
‘Let every Successor give back a little of what it took to build their honoured Chapters – in vessels, weapons, battle-brothers and plate. And for Dorn’s sake, let that plate be painted yellow.’
‘For Dorn’s sake?’ Issachar said. ‘For Dorn’s sake, let the best of us – victorious in the Feast and brother to all of us – take his rightful place as Master of the Imperial Fists. Let his name be carved into the bones of Rogal Dorn, and let no son of Dorn speak beyond our brotherhood of this again.’
•Abandoning the tradition of wall-names:
The Beheading, ch4
‘My third, and final decree,’ said Thane. ‘I will be a crusading lord. The Imperial Fists shall no longer stand upon a wall and look out on our enemies and defy them to come to us. We shall be a mobile fortress, moving ever forward to crush those who would spurn the will of the Emperor, whether they be known or unknown. Dangers cannot be waited upon, but must be rooted out and destroyed ere they pose a risk. Never again shall an enemy of the Imperium enter the orbit of Holy Terra. This we swear! For what use is a wall if all the lands about it are laid waste and occupied by the hateful foe? No longer will we stand in one place. No longer will we take wall-names, but we shall become a wall for the whole Imperium. Phalanx will leave before the end of the week, and I will be taking the Imperial Fists with me.’
Admittedly I could not find any direct quote that the IF organization was changed from 50 Walls x 20 Marines to the standard 10 companies x 100 Marines, but this timeframe seems plausible to me, as
Thane was previously CM of the Fists Exemplar - the hardcore Codex-compliant chapter among the IF's Second Founding successors, and he folded its remnants into the new IF.