Tom
 The DCAs (Decimal Counter Assembly) in those counters (5243 series, 5245
 series) came in low and high speed versions. The -6016 is the normal low
 speed 600kHz version used in most of the count positions and provides +1224
 BCD output to the rear panel connector.  The -6003 version is the option 03
 variation of it  that provided 1248 BCD output with the '1' state
 negative.(as opposed to the -6002 which was the same but with '1' state
 positive.)
 I have schematics if you need 'em- my 5245M still rolls along quite nicely
 since it knows I have the service info! 
Yes, after I posted my reply last night, I noticed the appendix in the
5245 manual that mentioned the 1248 BCD options...
AFAIK the counters are plug-compatible apart from the code used on the
BCD output lines (and internally on the counter PCB, of course). So you
may not need to worry about which version you actually have. The
schematics are also very similar...
-tony