Been trying to understand the NCO rank structure WW1 and post WW1 for years !! Many thanks to FROGSMILE for a detailed explanation.

Source: Royal Regiment of Artillery Rank

The infantry and cavalry had no 'substantive' rank (that is giving seniority and pension rights) for the level of junior non-commissioned officer with one stripe, whereas the technical Corps, RA, RE and AOC, did. This remained the case until 1961 when the Lance Corporal was at last made a rank, rather than an appointment.

Incidentally, a Lance Sergeant was in the same position. His substantive rank was Corporal, but he held the appointment of Sergeant as a qualified aspirant awaiting a vacancy. As these men did receive a pay increment, the COs of infantry battalions had an 'allowance' that they were not to exceed unless they wanted War Office/Department accountants breathing ire down their necks.

Ergo, the bombardier was only an 'equivalent' in ONE area and that was in wearing one stripe on his arm. The Bombardier held a pension and seniority earning rank that could not be removed except by court martial. The Lance Corporal held an appointment that was the gift of his commanding officer and that could be removed instantly via the same means.

As well as 'Bombardier' (one stripe) the RA actively and formally used 'Acting Bombardier' (also one stripe), whose status was exactly that of a Lance Corporal in the infantry, in that the position was an appointment and not a rank and was granted and taken away at the CO's whim. It was utilised in exactly the same way as Lance Corporal.

In 1920 existing RA Corporals (2 stripes) became Lance Sergeants (3 stripes), Bombardiers were all given 2 stripes and a new appointment of lance-bombardier (1 stripe) was introduced, thus bringing arrangements in line with the infantry and line cavalry.

Source: Royal Regiment of Artillery Rank