The "V8" mandatory version was 3.6.0.1. So user nounam is ok. Versionstring 3.5.9.0a-553-g2e58af1 has git commit hash in it, 2e58af1, which is the latest leisure release.

Apparently 3.6.0.2 was marked mandatory too. Let me explain. 3.6.0.1 had all the necessary things for v8, but the height at which first v8 block were accepted was erroneously set to 1010001 while the same version would begin creating v8 blocks at height 1010000. V8 was formally scheduled for 1010000. There were two alternatives to fix this. Either we change accept block to 10100000, which would require all exchanges and whales who already updated to .1 release to update again. Or move the miner block and the start of V8 one block up. The later change was released as 3.6.0.2. The only downside was that 3.6.0.1 nodes were not able to stake block 1010000.