Found a few sources for this and a comment citing 55 as the level cap, not 51 that it was later changed to.
Source: Casters realm spell entry
Date: May 2001 (data matches until well after timeline)
http://web.archive.org/web/200105220...ll.asp?Id=1057
Comment from the same link
Date: Aug 2001
"LEVEL 55 MOBS, By Tarbaand (6/28/2001)
This song can charm creatures up to level 55. Charm is an incredibly powerful ability, and we can charm creatures up to level 39 for free. A fiftieth level bard with no gear that boosts mana will probably have around 700 mana. Thats 35 charms, and if they last an average of 5 seconds (including resists). Consider the devestation a lvl 55 mob can dish out in 3 minutes, and consider that being done to your enemies instead of your party. No we cant do it often, but its not like we have anything else to spend our mana on"
Source: EQDiva
Date: April 2001
https://web.archive.org/web/20010420...gs.asp?song=41
"Description: Charms target up to 18 seconds. Lowers target's magic resistance. This is the upgrade to Solon's Song of the Sirens. Only works on mobs below level 55."
Going to need to do some more source work on this one I think as there is nothing I see in the SPDATs that references there is a level restriction.