Biggles
This target you are referring to is not the standard NSRA 6 yard target
There have been 3 different 6 yard targets used in competition over the past 30 years and the current one used in NSRA competitions which has superceeded the previous 2 (easier ones) is called the AIR 8 Target
It has been used since 1980 in NSRA run air pistol 6 yard competitions in the UK
The diameter of the 1 ring is 83 mm
The diameter of the 2 ring is 74 mm
The diameter of the 7 ring is 31mm
The diameter of the bull is 4.5mm
This target also has a Zero ring which is 92 mm in diameter
This target is a scaled down version of the 10M target and most shooters will get similar scores at both 10M and 6yds using the appropriate targets
I would guess that the target you have described may be based on the dimensions of the 1970's NARPA cards. NARPA dissappeared back in the 1980's
I hope this helps.
Unfortunately I have only seen 1 source for purchasing these 6 yard cards and that of course is the NSRA
John