Y09 means one of two things,, When it was made or the case and terminal style, Ignore that and go with the 6 Amp.
NOTE: 12 volts is a "Minimum" rating, if you get one rated for 24 or 48, that's good too. You won't find one rated for less than 12 so don't worry about that.
There are basically only a few factors other than the case/terminal arrangement
Max voltage
Trip current
Delay
Delay is flexible on most systems, and often not specified
Self resetting/Manual reset
And "Hold off resetting till current stops" (I think the Shortstop line does this, once it tripps it stays tripped till power is removed or the short(all loads) are removed).