curl 7.42.1 (i370-ibm-openedition)
Accessing ftp://service.boulder.ibm.com/s390/holddata/month.txt
by an HTTP proxy through a firewall, for example:

http_proxy=http://redacted.com:80
ALL_PROXY="$http_proxy" export http_proxy ALL_PROXY

The data arrive so that:

The first few thousand bytes when translated from EBCDIC to
ASCII (that’s right!) appear as EBCDIC. The remainder of the
file is good EBCDIC.

curl -q 'ftp://service.boulder.ibm.com/s390/holddata/month.txt;type=I’
curl -q ‘ftp://service.boulder.ibm.com/s390/holddata/month.txt;type=A’
… produce identical bad results.

– gil