4250 input buffer limit?

I'm trying to add a public key for SSH, but I appear to be hitting a limit on the number of characters allowed at once.

 

On the acme:

# SSH-pub-key import authorized-key rancid

works just great, and asks me to paste in the SSH key.  On all the 4500 units, this works with no problem, but on all the 4250s, only half the key is shown on screen when I paste it, then the SSH session freezes and drops with "Write failed: Broken pipe"

 

I've tried breaking up the SSH key as I paste it in, but again it will only accept 286 characters out of the total of 436 characters.