Transmission Control Unit

500 is for vehicles manufactured from feb, 2010 to Jul, 2010. 501 is for vehicles manufactured from Jul, 2010 to Mar, 2013. Goto jp-carparts.com and type the parts #s and you'll see.

Can I substitute one for the other?

I don't think the 501 will work,won't communicate with computer and if you install it, they will not refund you your money

try to figure out which model you have, the b13 USA is for like "03-". If you'd like to select a different model, just delete the "us_b13/" and see what I'm talking about. If you can find that transmission control, it will give you a range and whether or not it's accepted. I found one for a 25i that was like 2008ish and then I replaced the part # in the link with both of the ones you have and they came back as 404 file not found. Hope you figure it out. http://opposedforces.com/parts/legacy/us_b13/

I think Irod is right.. http://jp-carparts.com/subaru/partlist.php? maker=subaru&car_baseid=F23000&type=B14&cartype=114&fig_i d=184