What kind of aircraft do you see sending only altitude? I assume those are helicopters or light aircraft that don't have other information than altitude available to the transponder. In such case the transponder sends out altitude only on ADS-B (and maybe a few additional status messages).
This is the decoded data I'm getting for the a/c sending only altitude. Not all of them are like this, but most are. They have to be jets at least for being at FL340 and sending a Vmax code
A6CFFC Extended-squitt. United States 33975ft * CA:L2-air
A6CFFC Short-air-sourv. United States AC:33975ft Vmax:300..600kts
A6CFFC Extended-squitt. United States 34000ft * CA:L2-air
A6CFFC All-call-reply United States CA:L2-air