Here's a few weapons from BECM D&D with what you get for having various levels of Mastery with them.
First, the rather nice two-handed sword
Basic, 1 attack, 1d10 damage, no special abilities
Skilled, 2d6+1 damage, stun and deflect(1) - stun forces a saving throw vs death ray to avoid being stunned which means 1/3 movement and no attacks or casting spells, Deflect allows you to attempt a saving throw against an attack against you to negate damage that number of times
Expert, 2d8, stun, Deflect (2)
Master, 1st attack, 3d6+3, 2nd attack, 2d6+3, styn, deflect (2)
Grandmaster, 4d6+6, 3d6+2, stun, deflect (3)
Now the Mace
BS, 1d6
SK, 2d4, -1AC/1 - you improve your AC by 1 against 1 attack each round
EX, 2d4+2, -2AC/2, thrown attack at -/10/20 feet
MS, 2d4+4, -3AC/3, -/10/20 range
GM, 2d4+6, -4Ac/3, 10/20/30 range
And a missile weapon, the Longbow
BS, 1d6, range 70/140/210 feet
SK, 1d8+1, 90/150/220. -1AC/1, delay (make a save or lose initiative next round)
EX, 1d10+2, 110/170/230, -2AC/1, delay
MS 3d6, second attack 1d10+4, 130/180/240, -2AC/2, delay
GM, 4d4+2, 1d10+6, 150/200/250, -2AC/2, delay
There were other effects too, like the Flourish which was basically an AoE demoralise common to all masters. Other weapons had some other SFX attached. Any character could try and get mastery by spending a proficiency slot and getting training, though it wasn't automatic you'd succeed. The reason some entries give a second attack is because in BECMI you normally only got one attack each round. And it was an optional system so I don't know how many groups playing at higher levels (it was introduced in the later sets) used it.
Not sure that the 1D&D weapon master quite holds up so well.