According to Oman, when Mortier originally retreated from Extremadura in March 1810 the divisions of Ballasteros and Contreras were detached south to threaten the borders of Andalusia (Oman 1996a p.215).

Contreras 'Epitome' confirms this but doesn't give any information about the units in the division under Contreras command. It does mention that Contreras was given command by the Duque d'Albuquerque of a detachment that was sent to the Tagus. Oman confirms that d'Albuquerque left a detachment behind on the Tagus (Oman 1996a p.111).

His division must have been a different one from his command referenced earlier in the book (which consisted of Murcia, Siguenza, Real Marina, Reina) because these units appear at Ocana and therefore must have been part of the units d'Albuquerque sent to join the army of la Mancha.

So this means that Contreras division sent south with Ballasteros must consist of those units that formed part of d'Albuquerque's army before it left for Andalusia (and ultimately Cadiz) that got left behind when he left.

According to Oman, d'Albuquerque's army was split from Cuesta's original army of Extremadura. His army consisted of (Oman 1996 p.528):

• Reales Guardias Walonas
• Osuna
• Velez Malaga
• Voluntarios Extranjeros
• Provincial de Burgos
• Tiradores de Merida
• Provincial de Truxillo
• 2nd de Antequera
• Provincial de Badajoz
• Leales de Fernando VII
• Voluntarios de Plasencia
• Voluntarios de Zafra
• Voluntarios de La Serena

So which of these did d'Albuquerque leave behind? Nafziger's list (810ISAS) of the army subsequently in Cadiz (which presumably included all of d'Albuquerque's troops that left Extremadura ) shows only (with a bit of name variation):

• Reales Guardias Walonas
• Provincial de Truxillo
• Leales de Fernando VII
• Provincial de Badajoz
• Velez de Malaga
• Voluntarios de Plasencia
• Provincial de Burgos

I think it's fair to assume that the troops detached are likely to have ended up at some point in Badajoz (in fact the Epitome references Contreras being around Badajoz). So if we compare the subsequent garrison of Badajoz we have (Oman 1996b p.610:

• 2nd de Majorca
• 1st de Badajoz
• Provincial de Truxillo
• Provincial de Plasencia

Perhaps the duplicated regiments here are the result of several battalions being separated?

Finally, if we look at the Mendizabal's army at Gebora, this is the army that was based in Extremadura, so is long after the departure of d'Albuquerque, and also after the arrival of La Romana's army from Asturias. So any regiments that are in Mendizabal's army at Gebora that are also in d'Albuquerque's army of Extremadura from 1809 are probably regiments that got left behind in Extremadura when d'Albuquerque left for Cadiz. That list from Mendizabal is (Oman 1996b p.611):

• Osuna
• Zafra
• La Serena
• Fernando VII (likely there are multiple Fernando VII regiments though)

Maria Teresa Menguiano Romero (p54) agrees that Contreras was part of the March 1810 movement towards Andalusia. She says that Contreras commanded the first division of the Army of the Left. This is completely different from the assumptions I made above, consisting of:

That's potentially 11 batallions.

Oman, Charles A History of the Peninsular War Volume 3 (Greenhill Books, London, 1996a)

Oman, Charles A History of the Peninsular War Volume 4 (Greenhill Books, London, 1996b)

Epitome de la Historia Militar del General Don Juan Senen de Contreras__ (London, 1810)

Romero, Maria Teresa Menguiano La Guerra de la Independencia en la Sierra de Huelva (Diputacion Provincial de Huelva, 2017)