Originally Posted by
Johnnie_wad
As noted, it is a thumb shifter from Microshift.
No mention of thumbs in your OP.
Originally Posted by
Johnnie_wad
And as suggested, the #1 location on the shifter relates to the smallest ring on the front Chian-ring set.
I meant to add - if you're just replacing the shifter and cable then the only adjusting you should need to do is cable tension - the limit screws on the derailleur should already be set (assuming it was working properly before the shifter failed).