Short answer
If the empty cell or manual says SR43 / 301 / 386, buy silver oxide — not alkaline LR43 / AG12, even when the pack “fits.”
If it says LR43 / AG12 / LR1142 / 186, you want alkaline. Same diameter and height; different chemistry and discharge behaviour.
Same size, different chemistry
Both are about 11.6 × 4.2 mm button cells. LR43 is alkaline (~1.5 V). SR43 is silver oxide (~1.55 V).
A device that names SR43 / 301 may run poorly or briefly on alkaline LR43 even when the cell fits. Prefer the chemistry the maker specifies.
Pack labels that cause the mix-up
AG12 / LR1142 / 186 usually mean alkaline LR43 — labelling, not a second size.
301 / 386 usually mean silver-oxide SR43. Charts that print “LR43 = 301 = AG12” as one battery mix chemistries; we keep them separate.
Do not confuse with LR44 / SR44
LR44 / SR44 share the ~11.6 mm diameter but are taller (~5.4 mm vs ~4.2 mm). They are not drop-ins for SR43 / LR43 — see LR44 vs SR44.
Do not use list
Do not treat 301 / SR43 as an LR43 / AG12 equivalent (or the reverse) without checking the device.
Do not confuse either cell with taller LR44 / SR44 or thinner SR1120SW / LR55 cells in the same diameter family.