MK1 and MK2 mags are the same mag except for what side the thumb button is on. They are all reversable except for the origanal MK1 mags. They are all sent in the MK2 configuration. Just remove the base plat slide the button out and put it in the other side and put the base plate back on and your good to go.
Ruger Standard and Mark I target magazines are
not the same as the Mark II.
Std and Mark I target use a 9 round magazine with a chrome, non-removable base.
Beginning with the Mark II series of pistols they changed to a style with a removable base and 10 round capacity.
If the OP's gun is truly a Mark I, or Standard model as the fixed sighted versions were called, then he needs the magazine that has a shiny base and holds only 9 rounds.
If it's a Mark II then the base is black plastic, removable, and it has a 10 round capacity.
Of course there were after market mags for both Std/Mark I and Mark II that may be different so this only applies to the factory magazines.