Background

Below a table showing how the Italian 21 Army Corps was structured for the attack on Tobruk, dated 18 November 1941. My thanks to Michele who found this in the Italian archives.

XXI Army Corps

XXI Corpo Armata was commanded by General Enea Navarini, who unlike many other Italian commanders remained with his command from 1941 to 1943. It was a relatively powerful formation, with over 200 field and medium guns, 19 rifle battalions and a good number of support battalions. The Corps participated in all the major battles of North Africa.

For the planned attack on Tobruk, the Corps was subordinated to Panzergruppe Afrika, together with the D.A.K., and it was supposed to play a crucial role in the attack, providing two infantry divisions and specialized assault engineers for the initial break into the fortress defenses..

It had been reinforced for this, e.g. by the addition of the 31^st Assault Engineer Battalion. It is however of note that while it had a lot of guns, most of them were quite out of date, with design dates for guns ranging back to 1890. Furthermore, the light guns of the Italian infantry divisions simply could not deliver the weight of fire and/or range required to support typical combat in World War 2. Nevertheless, it had substantial firepower, and if a troop of British tanks were to encounter the truck-mounted naval 76/40 guns, it probably wouldn’t have made the tank commanders feel better to know that it was first designed in 1898.

A WW1 vintage 152/37mm heavy gun being transported on a trailer in North Africa. This was a standard mode of transport for guns which had not been modernised with rubber wheels between the wars. Note that this form of transport did nothing to the mobility and agility of the guns. Source: Wikipedia

Italian Artillery Positions, Tobruk, 18 Nov 1941. Roman numerals are the Group, Arabic the Regiment. Trento and Bologna sectors only. Red line indicates position of main defensive line. Source: Attachments to War Diary of Corps Artillery Command, 21 Army Corps (Click on the link to open better picture in new window)

Artillery OOB

While it may appear weak in anti-tank (47mm Boehler) and anti-air (20mm Breda) guns, these were part of the support battalions in infantry regiments, so the Corps as a whole would have been reasonably well equipped with these weapons, although the Corps had only a very small floating reserve for allocation in critical situations, and none left once that had been parcelled out prior to the attack taking place.
It lacked armour however, and in fact for its role in the attack on Tobruk requested a German medium tank company in support. Nevertheless, as a operations in October around the seizure of the Bir el Azazi had shown, armour was quickly brought up from the frontier in support of operations around Tobruk.

A note on the number of guns – this is always an extrapolation by me, assuming 4-gun batteries (bty) were standard throughout. More importantly though, in a few cases I also had to make assumptions on the number of batteries in a group (I assume it’s always 3 batteries/group). I would welcome corrections on this. Some other questions I would be interested in are the number of companys (coys) and tanks in V Light Tank Battalion, and the number of companies in XXXI Assault Engineer Battalion. I also assumed that both Regia Marina units were equipped with truck-mounted guns.

Source: Attachments to the War Diary 21 Army Corps, Ufficio Storico Esercito

Command Conference, HQ of 21 Italian Army Corps, El Adem, 21 Nov. 1941, from left:
Seated: General Enea Navarini, Commander 21 Army Corps, General Rommel, Commander Panzergruppe Standing: Unknown, Colonel Diesener, German Liason Officer to 21 Corps, Unknown, General Gause, Chief of Staff Panzergruppe – Source: Bundesarchiv, Wikipedia