I wouldn't worry to much about substitutes as long as current raging is 
high enough.  As I mentioned earlier, I often replace with darlingtons, if 
I remember the number right, e c g 247 or e c g 249, which I think the 249 
is higher current.
I just looked up on lline and if I read things right, the 247 is rated 12 
amps and 249 16 amps.  Even numbers such as 248 and 250 are pnp versions 
of the same transister.  I don't recall the astrons having insolaters that 
go under the head of the screw, you have to make sure the socket is set in 
the proper holes, the insolater for the screws is usually made on the 
socket.  I always check with an ohm meter and make sure the colectors 
aren't shorted to the heat sink.
73
Butch
WA0VJR
Node 3148
Wallace, ks.