The MSD 6-series is an analogue captive-discharge ignition. That's what you wanted. The 6AL is the basic 6A box plus a "soft" random cylinder ignition cut rev limiter.

The MSD 5 is a simple analogue multi-spark system. Nothing really special.

If it was up to me, I'd have gotten a Crane Hi-6 or a Jacob's FC1000. Both of those are digital captive-discharge units.