On the contrast:
While I said I no longer have the problem now that I use the library, I suspect the real story is that the library just happens to have the settings OK with the information I now display. I had three non-library programmes and the only one that worked was Stuart LIttle's. I played around with the hex stuff to no avail. I then made Little's commands common to all three. It helped the others but not enough. It appears that the quality of the display is partly determined by the nature of the information displayed.
I think I got some info on contrast from this magnum opus.
http://ianlangelectronic.webeden.co.uk/#/lcd-module-0/4569058582
Note also:
I understand these devices are surplus, so their price may reflect their compliance with specification. In short, the contrast issue may be partly down to luck. I have two and they are both OK for my needs, but one has a blue backlight which is quite useless and better left off.