Before the Imperial Guard deployed to Devos IV, the Departmento Munitorium, always with a weather eye to the consumption of resources, looked at the units already in system.
The Preatorian 5th Regiment were deployed to the system to guard the System Defence Network; A reinforced company at the Astropath station on the fringes of the system and two Battalions spread amongst the moons and planetoids where the missile silos waited for intruders. And best of all, one battalion planetside, with proper air, farms, water and cities; rotated on an annual basis, the 'lucky battalion' would spend it's deployment ether exercising with the PDF or on R&R.
One Battalion was stationed on Devos IV. The regiment had been sent to the Devos system to recover after eighteen years on campaign. There was always the intention to send more draughts of recruits to Devos IV to join their new regiment; with a well regarded PDF to exercise with, there was a clear plan in place to bring the regiment back up to six battalions and ready to once more take to the stars. By the time the rebellion happens, each Battalion has had three years planetside, being well looked after.
Collaboration with the PDF and settled, quiet deployment for seven years, witnessing and becoming involved with a grateful population had an effect on the Preatorian 5th; when the call came, they chose to defend their adoptive home.
The Cadian 144th Long Range Recce Regt is a trail unit, formed from remnants of other specialist units, principally Kasarkin and Airborne Regiments. Having worked quite well, the 144th was enroute from Hydraphur to Cadia following active service. The half strength experimental unit, believed to be at just about battalion strength, was diverted to Devos IV in response to a request for a show of strength by the Ordo Hereticus.
By the time the survivors join 17 Korps, they are down to company strength. Gen Tolstoy tends to use them as a Korps Recce Asset, despite them being nominally a part of 903 Div.
The 2nd Devos IV Regiment of Foot was counted by the Departmento Munitorium as a Guard unit, rather than PDF as it was due to form the backbone of another founding for a Devos IV Imperial Guard Regiment. Although this process had begun in administrative terms, it had yet to have any effect outside of Klestor Subsector Command on Agripinna. Certainly the 2nd Regt (Gardia Presedentiale) weren't aware.
The Arcomet 887th, like the Cadian 144th were en route to their homeworld to be reinforced and refitted. 4* Gen Zhukov wanted Armour and Air Mobile units and Comptroller Bellormus was able to secure the 887th for 72AG; resistance to his indent for resources was overcome largely due to other bidders trying for full strength units. The 887th found themselves turning around a week into their homeward journey to deploy in the vanguard of 72AG. The two remaining Battalions of the 887th, one armoured and one air mobile, are generally used by Gen Zhukov as his immediate reserve or QRF.
The Valhallan (Kado) 540th is an experienced, recently rebuilt and retrained (in the Cadian pattern), well led (not something common for Valhallan regiments) and above all professional infantry regiment of six battalions. Which has proved fortuitous, as they are, effectively, 903 Div.
903 Inf Div was supposed to be led by Maj Gen Marcus Firth, a former Cadian Officer appointed as Planetary Governor eighteen years ago, was an obvious choice for the Departmento Munitorium. He is an experienced officer of the General Staff, with command experience and furthermore has extensive local knowledge. However, at the point at which the insurrection began he was abducted or gave himself up to persons unknown. He is now in Ordo Hereticus custody and command of 903 Inf Div has passed to his CoS, Gen Gelu (Pardus). Gen Gelu has been a staff officer for almost his entire career. He hasn't been in a command appointment since he was a subaltern, some sixty two years ago.
What this means for 17 Korps is that 903 Inf Div effectively does not exist. This means that the other Divisions within the Korps will be stretched, juggled and run around by Korps HQ in order to fulfill orders from Army Group HQ.