Motorcycles in the 60's and 70's that didn't have electric start often had 6 volt batteries. I know I had a 1980 Yamaha DT-175 with a 6 volt battery. I can't remember if the 1984 RZ-350 had 6-volt or 12-volt.
All modern bikes with electric start have 12 volt batteries.
That charger looks like an excellent one to use to keep your motorcycle battery topped up. I use a Battery Tender that is about half as big and just plug it in when I won't be riding the bike for two weeks or more. (In other words, I hardly ever use it.)
Yeah, 1.5A won't hurt anything... and it may not help that much.
Charging is rated in C- for example, if you discharge a battery at 100 amps, then charging it at 50 amps is C/2. C/1 would fry many types of batteries; most chargers are C/2 to C/4. C/10 is perfectly safe if you don't overcharge; C/20 sounds about right for trickle charging. C/50 may even be safe to leave on all the time as a "float" charge.
Charging is also time-dependent; you can charge faster at certain times without damaging the batteries. Sorry, don't remember exactly what those times are. That's the trick of fast chargers. They can feed a high current when the battery can take it. Then, when they sense the higher level of charge (using battery voltage), they back off. When the battery is close to full, they back all the way down to a maintenance current.
1.5A sounds on the low side, but not insignificant. I used 1A to recharge a dying Monster battery, not just top it off. But it took a while. Look at your battery's spec to see what 1.5A amounts to.
Of course, I'm a mechanical engineer, not an electrochemist, so I could be totally wrong.