To get a working key for the "new" PC versions of Metal Slug, the most secure and reliable method is to purchase the game through official digital storefronts. Where to Buy Metal Slug
The closest official “complete” collection is Metal Slug Anthology (MS1-6 + MS5). It was released for PS2, Wii, and PSP. On a powerful PC, you can legally rip your own disc and play it via PCSX2 (PS2 emulator). No serial number required. serial number metal slug complete pc new
If it came in a paper sleeve, the key is often printed on the flap. Common Issues with "New" PC Copies To get a working key for the "new"