no caliber given but this little toy probably fire the 12mm pinfire shotgun cartridge(36 ga/.410)
https://aukro.cz/puska-system-lefos-vzacne-skladaci-provedeni-kolem-1850-gravirovana-6961236357

a Thieme & Schlegelmilch/Suhl 12 ga, SN 36326 with the Nimrod action

https://www.lauritz.com/da/auktion/thiem...-kal/i5500574/#

maybe a 24 ga https://aukro.cz/revolverova-brokovnice-systemu-lefos-rarita-1863-75-6952347252
"Smooth barrel caliber 15mm, on the bar signature "SERRA". Six chamber cylinder, with side ejector bar. It has a walnut stock on the neck with a roughened dovetail. Frame and trigger guard decorated with floral engraving. Length is 115cm. Gunsmith "MARTIN SERRA" documented in 1863-1875 as gunsmith in HAVANA. Rotation and release mechanism OK. The stock has 5 holes for woodworm, but it is long gone ... holes can be retouched. (but I don’t offer a hare in a bag, so I introduce and do not retouch)"