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The 1st is likely the battalion number. The fourth is a bit of a puzzler. It may be the 4th Regiment of the Royal Scots. Some units were always numbered, regardless of logic. However, history tolerates the right way, the wrong way and the army way.

As an aside, there were some regiments (ie. badged regiments) that raised dozens of battalions that fed the giant yawning mah that was the Western Front, depots, replacements and remusters.