It is an HMT watch. an is used here because HMT is actually pronounced as 'ech-em-tee'. 'Ech' starts with 'e', and thus, 'an' is used in this sentence.
4
Shubhodiya Ghosh answered this
it will be it is an hmt watch the reason for 'an' and not 'a' is that when u pronounce 'H' it sounds like 'eh'. hope i helped:)