The small primer seater assembly is the priming unit dedicated to small diameter primers: the punch and cup work together to push the primer into the primer pocket at the end of the press stroke.
Dillon lists part number 21381 as "Primer Seater Assy SM" in the primer punch and casefeed body assembly of the XL 650, page 50 of the manual, paired with the large version, code 21380. When switching from a caliber using small primers to one using large primers, the assembly must be replaced along with the rest of the primer feed system.
Main features
Primer punch and cup assembly
Small version, for small diameter primers
Genuine Dillon Precision spare part for XL 650
Recommended use
Replacement of the priming assembly or switching to small primers on the Dillon XL 650 progressive press. For large primers, the assembly with Dillon code 21380 is required.