I thought it might be helpful for those members without Freeman's book to point out most of the differences for numerals that technically could be read upside down. The illustration below shows the main types we'll be looking at.
I have deliberately ignored the very first barred numeral types, mostly found on Sydney Views and Laureates.
The first 2R has 20 something rays as a rule (can be 17 to 19) and the second has 31 to 43 rays. The numerals in the second type are generally thicker and larger, but not always. The first 3R has 16 rays and the second has 12 rays. The 4B is just the numeral, a 4D is a duplex.
The list of numerals that can potentially be read upside down is:
6/9, 16/91, 18/81, 19/61, 66/99, 68/89, 86/98, 106/901, 108/801, 109/601, 116/911, 118/811, 119/611, 161/191, 166/991, 169/691, 186/981, 188/881, 189/681, 196/961, 198/861, 199/661, 606/909, 608/809, 616/919, 618/819, 666/999, 668/899, 669/699, 686/989, 688/889, 696/969, 698/869.
If I've missed any please let me know.
An easy way to distinguish a lot of these is the fact that the barred numeral type was the only one used from number 836 onwards.
Therefore, in every case but four, if the cancellation is a barred numeral it must be the higher of the two numbers, as the lower number never had this type. Two of the exceptions are 186 and 698, the barred numeral for these being common.
For 186/981 Freeman states the 981 has never been recorded. For 698/869 Freeman does not provide an image for 869 suggesting the cancellation is quite rare; it's rated RRR.
The other two are 119/611 (barred numerals for both are known but are very rare) and 669/699 (10mm gap between central bars on 669 compared to 9mm on 699).
The following numerals are only found in the 3R type with 12 rays. Their upside down counterparts are not found in this type at all: 601, 611, 661, 681, 691, 801 and 811
Some of the others can be distinguished as follows:
18/81 - 2R type, 18 has 17 rays, 81 has 33 rays
18/81 - 3R 16 rays type, the 8 in the 18 is very close to the rays, in the 81 the numerals are well centered in the rays
86/98 - 2R type, 86 has 36 rays, 98 has 20 rays (NB: numeral 86 with 20 rays was used at Gayndah in Queensland. The top loop of the 8 is smaller than the bottom in both the 86 and 98, making it easy to orientate the cancel)
618/819 - 3R 12 rays type, numerals are closer together in the former
608/809 - 3R 12 rays type, 0 in 608 more oval, more circular in 809
This leaves the following which aren't so easy:
66/99 & 161/191 - each has a 2R type
6/9 - both have a similar 3R 16 rays type
16/91 & 19/61 & 68/89 - each has a 2R and a 3R 16 rays type
For these, if you have any queries post up a scan and we should be able to identify them for you. Rarity, size and the position of the rays relative to the numerals are three ways to distinguish one from the other.
Hope all this helps, and makes some sense