OK got it - well spotted, Big Dave ...
I'll add TYOTG = TMAHM
The solution is they are the initials of the main title and alternate titles of the works of Gilbert & Sullivan, as follows:
I = TPATP
PI = CA
R = TWC
T = TGGO
TM = TTOT
TPOP = TSOD
P = BB
TG = TKOB
TYOTG = TMAHM
Iolanthe I = The Peer and the Peri TPAPT
Princess Ida PI = Castle Adamant CA
Ruddigore R = The Witch's Curse TWC
Thespis T = The Gods Grown Old TGGO
The Mikado TM = The Town of Titipu TTOT
The Pirates of Penzance TPOP = The Slave of Duty TSOD
Patience P = Bunthorne's Bride BB
The Gondoliers TG = The King of Barataria TKOB
The Yeoman of the Guard TYOTG = The Merryman and his Maid TMAHM