Looks like a full dress rank isignia for a 2nd lieut., or lieut., depending upon the date, for a Scootish regiment, given the thistle on the button.
Victorian crown, so pre-1902, and the M may well signify militia, prior to the forming of Territorial Army.
For most of the 19th century, 2nd lieut. rank isignia had no pip, with one for a lieut. amd two for a captain,instead of the one ,two,three pip isignia used since.
Looks like a Royal Scots Fusiliers Button to me.
Maybe the M for the Militia Bn. they had If I recall right. Later these became the TA Bn's in 1908 during the Childers Reforms.
Needs a check as I might be wrong.
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