What crank arm is this?
 

http://www.ebay.com/itm/160646626443...84.m1439.l2649

I impulse bid on this arm, hoping that it is a 50.4bcd as I have an extra outer ring for one. I can read forged duralumin on the arm, so it's got to be a quality arm if I can find a ring and a mismatched lefty.

The brand name seems to be Alero, with a ligature in the A+L to make it hard to read. You can see it here:

http://bmxmuseum.com/forsale/pc042597_blowup.jpg

110 BCD BMX perhaps?

No, open his link (while it's still fresh), it definitely looks like 50.4 (which would be good) or something not too far off (which would be useless).

I've got my finger's crossed. We'll see in a week.

Hoping for the best when buying on-line is one almost sure way to experience disappointment. Look at the listing and pictures and ask questions! Ask as many as you need to to ensure that you are going to get what you want.

That is the advice I give every one of my customers. I do not want an unhappy person at the other end of a transaction.

It's 50.4.