Port of the classic Arcade title, which is a modified Metal Slug 2 with extra additions such as a few enemies that are not in MS2. The PSX port, published by Agetec outside of Japan, features an extra mode that isn't present in any other console port, that consists of 20 short challenges with a specific objective to achieve. This mode is included right after playing through the regular mode. And for those who are wondering, the missing letters for the list of rescued POWs is not an emulation bug, for some weird reason Agetec forgot to check that aspect thoroughly during localization. The other mode, named 'Combat School' are just challenges in the form of replaying the main levels over and over again and you do not get anything rewarding for doing so.
As someone who used to play the Anthology edition on the PSP, Metal Slug series are pretty fun and the animation is smooth, especially for the titular tank that acts very sentient. It aged very well for a 90s game.