The early serial numbers of this set had a slightly different back to the later serial numbers. Note the $5 is higher above the word Telstra than on the other example and also the 5 is thicker.
This occurred on prefix numbers 1399, 1400, 1401, 1402 & 1427 only. Other prefix numbers in this set had the lower $5 amount.
The same applies
to the $10 & $20 cards, with the addition that these two cards also
changed the wording of "$10.00 worth of calls" to "$10.40 worth of calls"
and "$20.00 worth of calls" to "$20.80 worth of calls" respectively.
The breakdown is as follows:-
1399, 1400 00000-29999 High Thick $5, 30000-99999 Low Thin $5
1401, 1402 00000-19999 High Thick $10, 20000-99999 Low Thin $10
1427 00000-24999 High Thick $20, 25000-99999 Low Thin $20
Prefix numbers 1428-1434 all have the Low Thin values.
