Warthur
Black Flag's The First Four Years is a mean little companion piece to Damaged, collecting all of the band's single and EP releases preceding their classic debut. It actually works fairly well as an album in its own right - partly thanks to its modest 25 minute running time - and the evolution of the band from a mildly harder than average punk band to the hardcore punk demigods beginning to dabble in other musical territories they were on Damaged is intriguing to witness. I won't get too deep into the arguments over which vocalist is best because all of them have a broadly similar approach to spitting out the band's lyrics and lay the groundwork for Rollins to come in admirably.
If you are really addicted to alternate versions of songs from here and Damaged you can take a look at Everything Went Black, but for most I think this is the only significant compilation of pre-Rollins material you need - not least because the overlap of material between this and Damaged is comparatively slight.