An Allis family in India

One family that caught my attention recently was from India.
William Allis, a private with the 74th Highlanders, had a child with Katheriamal (there are a number of spellings) who he married shortly afterwards in Bellary, Karnataka, India. Katheriamal anglicised her name to Catherine.
They married in 1859 when William was said to be 24 and Catherine 20 years old. When he died in January 1882 William’s age was given as a precise 51 years and 10 months which suggests he was about 29 when he married.
Catherine died in 1887, age 47.

They had seven known children with one not surviving infancy.
The family continued to live in India and their descendants are probably still there today.

The origins of William are unknown except that his father was also William. There do not seem to be suitable candidates for William’s baptism in various records for Scotland or England.
The 74th Highlanders have their origins in Scotland and, from Wikipedia, were in Bermuda from 1828 until December 1829 when they returned home. There followed a tour from 1834 in the West Indies and Canada returning home in 1845. They then sailed for the Cape in 1851 moving to India in 1854.
The highlanders left India around 1864 but William had transferred to the Royal Artillery and stayed on taking an army pension.
It therefore seems likely that if his father was in the army he was born at home.

Any other angles on his origins would be appreciated.