SG Concise vs Scott's

In order of increasing depth for GB if comparing these 2 publishers:

SG Stamps of the World
Scott WW Standard
SG GB Concise
SG GB Specialised
...

there are plenty of printing issues with Machins. They were real workhorse issues and many printings on various sources (sheets, coils, booklet). I do not collect errors so do not care much. I had emailed you my super spreadsheet on Machins. Let me know if you want it again. As I noted above Scott lists two areas SG Concise does not - EME vs non-EME for same issue and some of the thin/thick values.

Missing phosphor, short phosphor bars, shifted bars. Then there are variations in papers, optical brighteners, phosphor coatings like ACP vs PCP1 vs PCP2. Unlike many modern issues, they are well studied. There is much less attention to modern issues in the US compare to the GB interest in Machins.  Scott ignores modern varieties for most countries except US.

Note that Michel also produces a Great Britain Specialized catalogue that in terms of depth is between Gibbons GB Concise and the 5 volume GB Specialized

to add to the list khj made, there is also Gibbons "Collect British Stamps" which is about same coverage as Scott's post 1952 GB listings (pre-1952 Scott is deeper in its Classic Worldwide specialized, though still not as deep as Gibbons GB Concise).

Scott for GB (and for anything other that USA) is always going to be much more basic vs a more Specialized catalogue of the country of origin. Just compare Scott's coverage of 1960s and 1970s Canada to that of Unitrade.

Re quality control let's not forget that multicolor offset lithography and photogravure were in their infancy in that era for stamp printing, so printers had a lot of learning to do and improvements to make, and it is only in the 1980s that these printing methods are stable to point where color registration, plate flaws, and color errors become more exception than the rule.