Vic,
The reason for the odd ball sizes is so you can ring these oddball sizes together to get a more complete check along the thread of the mike. That set does have one strange combination of the 6.5 and 12.5 which make up 19 and there is 19 block.
I used to have a Mitutoyo gauges for metric and imperial micrometers, they both had more gauges in them. From memory there was about 15 gauges in each set. I do remember that one set had 1 more gauge than the other set. The set you saw will allow checking faily comprehenively up to 75 mm and to a lesser degree 100mm. I no longer have the Britsih Standards for checking instruments with gauges so I cannot check but I doubt if the gauge set you saw would comply comply with BS.
Barry G