The 1934 series $10 Silver Certificates had micro-size front plates and back plates and the 1934A series had the opposite, macro-size face plates and back plates.  When a 1934 $10 Silver Certificate was mated with a newer macro-size back plate, it created a 1934 macro-back Mule.  When a 1934A $10 Silver Certificate was mated with an older micro-size back plate, it created a 1934A micro-back Mule.  The 1934 Mules are fairly common (except the ultra-rare B-A block Mules), whereas the 1934A Mules are much fewer and further between.

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Until recently, 1934A Mules are known to have been printed with only four micro back plates, BP #404, #523, #553, & #578. But a recent discovery by Jeff Blind (pictured below) changed that when he found a 1934A Mule with micro BP #583!  These five micro back plates are also found on 1934A $10 FRN Mules and 1934 $10 Silver Cert and FRN Non-Mules, along with many other micro back plates.