Is 321 the same as SR616SW?

Yes. 321 is another code for the same cell: silver oxide, 1.55 V, 6.8 × 1.65 mm.

Is SR65 the same as SR616SW?

Yes. SR65 is another code for the same cell: silver oxide, 1.55 V, 6.8 × 1.65 mm.

Same 6.8 mm diameter, but 2.1 mm tall instead of 1.65 mm. Only if the holder fits; thinner cells can lose contact.

Same 6.8 mm diameter, but 2.6 mm tall instead of 1.65 mm. Only if the holder fits; thinner cells can lose contact.

SR616SW is a 1.55 V silver oxide cell with a typical capacity of 14.5 mAh.

6.8 mm across and 1.65 mm thick.

SR616SW is the same cell as 321 and SR65, so any of those codes will fit; no other cell here is 6.8 × 1.65 mm at 1.5 V.

SR616SW at actual relative size: 6.8 mm across, 1.65 mm thick. The dashed circle is a US quarter, 24.26 mm across (US Mint coin specifications).

The same cell under another code. Any of these fits, whatever the brand.

Same diameter, different thickness. These are not drop-in replacements.

Cell	Size	Verdict
SR621SW	6.8 × 2.1 mm	Same 6.8 mm diameter, but 2.1 mm tall instead of 1.65 mm. Only if the holder fits; thinner cells can lose contact.
SR626SW	6.8 × 2.6 mm	Same 6.8 mm diameter, but 2.6 mm tall instead of 1.65 mm. Only if the holder fits; thinner cells can lose contact.

On the shelf it is sold with a SR616 on the pack .

Is 321 the same as SR616SW?: Yes. 321 is another code for the same cell: silver oxide, 1.55 V, 6.8 × 1.65 mm.
Is SR65 the same as SR616SW?: Yes. SR65 is another code for the same cell: silver oxide, 1.55 V, 6.8 × 1.65 mm.
Will SR621SW work in place of SR616SW?: Same 6.8 mm diameter, but 2.1 mm tall instead of 1.65 mm. Only if the holder fits; thinner cells can lose contact.
Will SR626SW work in place of SR616SW?: Same 6.8 mm diameter, but 2.6 mm tall instead of 1.65 mm. Only if the holder fits; thinner cells can lose contact.
What voltage is SR616SW?: SR616SW is a 1.55 V silver oxide cell with a typical capacity of 14.5 mAh.
What size is SR616SW?: 6.8 mm across and 1.65 mm thick.