Your large 58 mm diameter Swiss made Mercer, Compac or Tesa YR and Tesa YE dial indicator probably comes with a plastic flat back. Four large slotted screws hold it in place. If you buy the optional cast metal lug back, you simply remove the plastic back and screw on the metal lug back using the same four screws.
Compac and Mercer are identical because they're the same indicator, sold under two different brand names. Tesa uses many of the same parts. These metal backs will fit all Compac, Mercer, Tesa YR and Tesa YE indicators made in Switzerland. They will not fit the old Mercer indicators made in England.
Every manufacturer (Starrett, Teclock, Mitutoyo for example) has slightly different specs for their indicator backs. As a result, you will have to use the manufacturer's own. They are not interchangeable between manufacturers.
Mercer indicator with optional lug back installed. The flat back has been removed.
If you're not familiar with the difference between on-center and off-center lug backs, take a look at these diagrams. They speak words.
on-center (left) and off-center (right)
The hole in the lug is 6.4 mm which equals .25". The dimensions given are metric (in millimeters) and correspond to the large (Ø 58 mm) indicators shown above.
The smaller models, with 40 mm diameter bezels, such as Compac 353E have a correspondingly smaller back but still have the quarter inch hole. There is also another difference: The small indicators do not have removable plastic backs. The indicators have a solid case. The optional lug back mounts directly onto the case using fours screws (not included). When you buy the small model lug backs, you will also receive four screws for this purpose.