A few simplifications:

    c:"curl http://jsoftware.com/papers/50/50_"
    e:".htm;sleep 3"
    f:{("intro";"chrono";"pers";"fine";"ref";"ack";-2#"0",$x)0 50 51 52 53 54?x}
    {`4:c,f[x],e}'!55

The pattern of using find and indexing into a table with a default case is a nice way to get rid of lots of twisty conditionals.