I think the problem stems from trying to use prefixes for what should have been solved by proper forum (re-)organization: Fremantle, Harmattan and Sailfish posts should be in their own subsections, rather than desperately trying to solve the mess of chucking them all in one basket by forcing cryptic prefixes.

Even so, it might not be as bad. The word cryptic is a clue here: people might find Fremantle and Harmattan less confusing than M5 and M6.

Of course no system is idiot-proof. We have a Buy & Sell section and still get WTB or FS posts in General