Double horizontal magazine for the HDR50. The possibility of mounting on the top RIS rail allows you to place other accessories on the bottom (for example, a flashlight or laser holder - models available on my profile).
Remix model: https://cults3d.com/en/3d-model/various/double-magholder-for-umarex-hdr50-tr50-co2-revolver-slugmaker-s-bestt23-412
The author remixed this model.