Sounds like a delta tub spout (can't be sure, of course, without a picture)...
Take a look at this link to an older post...after click on link, scroll down to post #7 and review the pics....does yours have threaded adapter like the one in pic..?
http://www.askmehelpdesk.com/plumbin...on-200364.html
Otherwise, see if you can take a picture of spout and post it at the "go advanced" feature at posting box...
OR...
have you considered adapting over to a newer tub spout with diverter?? Should not be too hard to adapt over to something that will work for you. Let us know what you think...Mark