They are too big for apron mounts and also not a shape used.
They are instead
Roman equine harness mounts.
The diamond / lozenge /
rhomboid ones were used from 1st to 3rd century and are
Nicolay type A8 and B7; Aurrecoechea
type M.
The more rounded one with one stud on the back is lenticular (lens shaped), these date from mid-2nd to 3rd century.
Nicolay type B10; Aurrecoechea
type B, especially Huber & Kremer (2011), #1020-1022 and
Redzic (2013) #522b.
SC