Used Honda Lawnmower. Confused with model numbers? HRR, HRX, HRS, HRC, HRT, HRB?

Since there was another recent post about lawnmowers here, I figure this might be the right place to ask.

I've been looking for Honda mower's on craigslist, and came upon many different models. Honda's website doesn't have a clear comparison, and don't list all the variations I've seen. They only list four model series HRR, HRX, HRS, HRC with only very brief descriptions:

• HRR = Residential

• HRX = Premium Residential

• HRS = Side Discharge

• HRC = Commercial

What is the HRT & HRB series? There's also HRZ. If they're not listed on their website, does that mean it's discontinued? It's hard to get a good idea of what a good price is if I don't know what features I'm comparing.