Canadian Postage Dues Post 1967

I was searching through some used copies of these stamps I recently purchased.

I have Stanley Gibbons, Scott and the Chung-Reiche book of 1985 to use as references.

As I understand the listings, the 1967 issue (the fifth issue) was perf 12 x 12 and the design was 21mm x 17mm in size. The sixth issue was perf 12 x 12 and the design was 20mm x 15.5mm. The gum used was dextrine. The seventh series was identical to the sixthth series with the exception of using PVA gum. The eighth series was perf 12.5 x 12.

My question is to whether you can distingish the sixth and seventh series in used copies given that gum was the only difference?