Short answer
If the movement or empty cell says 395 / 399 / SR927SW, buy silver oxide — not alkaline LR57 / AG7.
Trade numbers → IEC codes (with alkaline twins listed separately) are on the watch battery chart under Tools.
Same size, different chemistry
Both are about 9.5 × 2.7 mm button cells. LR57 (also AG7 / LR926) is alkaline (~1.5 V) with a sloping discharge curve.
SR927SW is silver oxide (~1.55 V), often printed 395 / 399 on retail packs. Quartz watches that name 395 / SR927SW expect the flatter silver-oxide curve.
Pack labels that cause the mix-up
AG7 / LR926 usually mean alkaline LR57 — labelling, not a second size.
395 / 399 usually mean silver-oxide SR927SW. Charts that print “AG7 = 395 = LR57 = SR927SW” as one battery are wrong — we keep alkaline and silver oxide separate.
Do not use list
Do not treat 395 / SR927SW as an LR57 / AG7 equivalent (or the reverse) without checking the device.
Do not buy from a flattened “395 / AG7” blister without reading which chemistry the pack actually is.
Next steps on this site
Start at the watch battery chart or Casio watch battery finder when the caseback only shows a trade number.
If the watch has stopped and you are unsure it even needs a cell, see Why has my watch stopped? before buying.