Expanded Ability Scores for the Holmes Ref

EXPANDED ABILITY SCORES (OPTIONAL) 

For all characters:

Strength 

15 or more: +1 to hit, damage and open doors

7 to 14: no bonus

6 or less: -1 to hit, damage and open doors

From Gygax's OD&D House Rules, compiled here; also in line with the bonuses for NPCs mentioned in the Holmes Basic version of Keep in the Borderlands (e.g. the imprisoned Hero in the Caves of Chaos gets a +2 to hit and damage due to level and 18 strength). Also not far off from that used in Warlock, which Holmes used prior to editing the Basic rules. In Warlock Str 16+ gives a +1 to hit, and Str 13-17 gives +1 damage, 18 +2 damage.

Intelligence

11 or more: one extra language per point over 10

10 or less: no extra languages

From the rules as written

Wisdom

15 or more: +1 to saving throws versus mental attacks (charm, fear, illusion, hold, etc)

7 to 14: no bonus

6 or less: -1 to saving throws versus mental attacks (charm, fear, illusion, hold etc)

Extrapolated based on the otherwise unexplained Wisdom Adj on the 1977 OD&D character sheet and the later AD&D PHB. Uses the same range as Gygax's house rules, where 15+ gains a bonus.

Constitution

18: add 3 to each hit die

17: add 2 to each hit die

15 to 16: add 1 to each hit die

7 to 14: no bonus

6 or less: subtract one from each hit die but never less than 1

From the rules as written

Dexterity 

13 or more:  fire any missile at +1

9 to 12: no bonus

8 or less: fire any missile at -1

From the rules as written

Charisma

18: up to 12 followers, +4 reaction rolls

16 to 17: up to 7 followers, +2 reaction rolls

13 to 15: up to 5 followers, +1 reaction rolls

10 to 12: up to 5 followers
7 to 9: up to 3 followers
5-6: up to 2 followers, -1 reaction rolls
3-4: up to 1 follower, -2 reaction rolls

In the Holmes Basic rulebook, page 5, Holmes writes "A character of charisma below 13 can not hire more than 5 followers, and their loyalty will be luke-warm at best — that is, if the fighting gets hot there is a good probability they will run away. On the other hand, someone with a charisma of 18 can win over a large number of followers (men or monsters) who will probably stand by him to the death." 

This is a reference to the charisma table in OD&D Vol 1, page 11. There is a slight discrepancy as in the OD&D table a score of 10-12 gets 4 followers, not 5.

Things get a little complicated after this. The OD&D Charisma table also has bonuses for loyalty for the these followers, which modifies a loyalty score on page 13, which in turn modifies morale, which is not explained very clearly. Holmes didn't include the rules for loyalty scores or morale, perhaps due to their complexity. 

However, the text explaining the reaction table on page 12 of OD&D Vol 1, states that the roll is "adjust[ed] for charisma", which seems to indicate that the same charisma modifiers are also used with this table. Holmes included a reference to this in Basic on page 11, where the text explaining the reaction table there sasy "The DM should make adjustments if the party spokesman has high charisma or offers other special inducements". Hence for our expanded table here, I've converted the loyalty bonuses to reaction adjustments. Moldvay treated this similarly in his version of Basic.