Hi Glen:
The OSCCAL value is stored at the top location (0x1FF) in an
unused 12C508-JW.  If it has been erased, then the best thing
to do is just program it for the midpoint value of 0x70.

;-------------------------------
        ORG        0x1FF    ;
        MOVLW      0x70     ; Actual value (0x00 - 0xF0) goes here
        ORG        0x000    ;
        MOVWF      OSCCAL   ;
          .          .      ;
          .          .      ;
          .          .      ;
        END                 ;
;------------------------------

All that the OSCCAL value does is to adjust the frequency of the
internal oscillator.  The factory value is chosen for each part
to compensate for varying RC values so that the internal osc.
will run close to 4 MHz.  If you have lost the original OSCCAL
value it isn't a big deal unless your application is frequency
critical.  If it is, you can find the value empirically.

Hope this helps,
 - - - Nick - - -