Waltham Model 72 Screw?

Would anyone know what the small silver screw on this movement is for...Just below the ratchet wheel and next to the plate screw.. Serial # 670287..
Pin set, is that odd ???

Thanks,
John

John,
That would be a let down arbor on the 1872 models.
Brian C.

The let down screw disappears later in the 1872 model. This open face example shows it relocated.

This later example shows it disappeared.