you can use window names for this, as they pass from iframe tag to iframe context.
parent doc:
<iframe name=fr2 src="data:text/html,%3Chtml%3E%0A%20%3Cscript%3E%20parent.postMessage%28%7Bname%3A%20window.name%7D%2C%20%22*%22%29%3B%3C/script%3E%0A%3C/html%3E"></iframe>
<iframe name=fr3 src="data:text/html,%3Chtml%3E%0A%20%3Cscript%3E%20parent.postMessage%28%7Bname%3A%20name%7D%2C%20%22*%22%29%3B%3C/script%3E%0A%3C/html%3E"></iframe>
<script>onmessage = function(e){ // use real event handlers in production
alert("from frame: " + e.data.name);
};</script>
iframe doc:
<html>
<script> parent.postMessage({name: name}, "*");</script>
</html>
which alerts "fr2", then "fr3". you can then easily use the name attrib to find the iframe in the parent DOM using attrib CSS selectors.
illustrative demo of window.name+iframe concept: http://pagedemos.com/namingframes/
this painfully simple approach is also immune to issues arising from same-url iframes.