I may be biased, haha, but in our old RM2 games I’d allow Dir. Spells to be developed as a base skill with no tag for what it was attached to.
Akin to how one develops Missile Weapon Category Skill Ranks in RMSS and then later adds to these with individual named weapon ranks like Sling/Longbow/Heavy X-Bow.
I would then allow half this bonus to be applied to any and all directed spells that the player would possess, as a base OB.
As a player reaches a level that allows more particular spells to be cast(Waterbolt, Firebolt, etc.), I would allow one to add this base OB to any new ranks developed, listed as Dir. Spells - Fire Bolt; Dir. Spells - Ice Bolt.
So 2x ranks/lvl developed until 4th level of the non-named base skill = +50 total bonus...remember Adolescence ranks
All Directed Spells ever known and also able to be cast without ESF will now get a +25 as a base OB(think as this having a -25 applied for not having the named skill, itself, developed).
Once LVL 2 is reached and if the PC has access to the spell in the first place, the spell user develops 2x ranks in “Dir. Spells - Shock Bolt” and now has an additional +30 to add to the original base OB of 25.
Just getting access to Water Bolt @ LVL 4 only allows 2x total ranks to be developed, Total, and so it now only gets +10 to add to. The base OB of 25.
*=*=*
To answer your question, though, any named spell that merely added a number to itself is the same spell and was allowed to use the original base “named” directed spell bonus.
So all Shock Bolts(n) would get the full Dir. Spell - Shock Bolt OB.