In addition to the Rifle Regiments, the Fusiliers did not have the rank of Ensign. The 7th Fusiliers, the 21st Fusiliers and 23rd Fusiliers had 1st Lieutenants and 2nd Lieutenants. The army lists would confirm this.
"Until 1871 the lowest commissioned rank was the Ensign in the Infantry and Cornet in the Cavalry - both names derived from French words signifying standard bearers. The Fusilier regiments, having no company colours, had First and Second Lieutenants anyway. The Fusiliers abolished the rank of Second Lieutenant in 1834. Between 1871-1877 the lowest was the Sub Lieutenant, after which today's Second Lieutenant rank was established. Second Lieutenant is the lowest commissioned rank in many armed forces."
"The rank was introduced throughout the British Army in 1871 to replace the rank of Ensign (Cornet in the cavalry), although it had long been used in the Royal Artillery, Royal Engineers and Fusilier regiments. At first the rank bore no distinct insignia. In 1902 a single star (sometimes referred to as a pip) was introduced; the ranks of Lieutenant and Captain had their number of stars increased by one to (respectively) two and three. The rank is also used by the Royal Marines."
"Terms such as second lieutenant are appointments not ranks and an individual officer could change from second to first lieutenant and back again without any promotion or demotion, depending on casualties or new postings to the company. The term first or second lieutenant was merely a reflection of the seniority of the individual officers and not a rank as such."
I would have to check that against the army lists, fairly certain though that 2nd Lieutenant at the time was a rank and promotion to 1st Lieutenant changed the seniority date of the individual in the regiment. If this was not so, then a Lieutenant in a Fusilier or Rifle Regiment had an advantage over an Ensign in a line regiment for exchanges.
From the Connaught Journal Sept. 1823, "22d (sic) Ditto- Lieut. Robert Pattison Holmes to be Captain by purchase, vice England promoted to the 49th Foot. Second Lieutenant Edmond Thos. Ellis, to be First Lieutenant, by purchase, vice Holmes- Wm. le Mesurier Tupper, Gent. to be Second Lieutenant, by purchase, vice Ellis." [Captain Richard England 23rd had purchased the Majority in the 49th]
Something to track down.