With all due respect, anyone who wanders the hills and does not appreciate the purpose and significance of 'grid square letters' on an Ordnance Survey map, should perhaps consider taking up something less intellectually demanding and dangerous like knitting. I have OS maps going back to 1901 and they are in the same format as ones of today.

Each set of sheet letters represents a 100 km square
SQ SR SS ST SU TQ TR
(01) (11) (21) (31) (41) (51) (61)
SV SW SX SY SZ TV
(00) (10) (20) (30) (40) (50)
The origin of the Grid is at the South West corner of 100 km square SV
Hungerford Railway station is at SU 341 685. This becomes
Easting 4321 Northing 1685 ie 432 km East of origin & 165 km North of the Origin. My one inch scale maps dating back to 1952 show the 100 km square grid across the whole of the UK

The sheet letters are very handy (essential) when determining the location of Check Points
Rather strangely the CP list for the 2013 100 does not give this information. I use
streetmap.com to assist me in getting to CP for marshal duty
For Camel Teign 100 the start in Wadebridge is in area SW and all the rest going eastwards are in area SX
Prior to the two letter code for the 100 km squares there was a cartesian co ordinate scheme. This can still be seen in the co ordinates calculated by STREETMAP

It's not just events. I have the same chore of going through the details before posting our group social walks. The problem is the walk leader thinks localy so just puts in the six figure number and doesn't bother with the two letter suffix which makes the position unique nationwide.

The events listing has a check on it so that you have to put the letters on. A grid reference is not complete without them - just the numbers are not unique. I have to do a bit of investigation sometimes before the event can go in the system.

Please forgive an old man's ignorance but what do you mean by "map sheet letters"? The only letters most normal folk use when giving a grid reference are those which identify the 100 000 metre square in which the subject point lies and I can't find any events advertised in the current and recent editions of STRIDER which fail to quote them.

Why don't some Event organisers put the map sheet letters with their published grid references for their walks? I find this annoying.