What is "airspace underlying a Class B airspace area designated for an airport"? For the purposes of airspeed restrictions within airspace, the FAA regulations list the following area description (emphasis mine):

14 CFR 91.117 (C)


  No person may operate an aircraft in the airspace underlying a Class B airspace area designated for an airport [...] at an indicated airspeed of more than 200 knots (230 mph).


Should we understand the described airspace to be any and all airspace underlying a Class B where that specific Class B is designated for an airport? Or, should we understand the described airspace to be any and all airspace designated for an airport (Class C, D, or E) which is underlying a Class B?

Are there letters of interpretation or other regulations which clarify the grammatical ambiguity of this regulation?
Yes, there is a legal interpretation on this:


  [...] under  the  express  language  of  the  regulation,  aircraft 
  operating  in  the  airspace  underlying  Class  B  airspace -
  irrespective  of  whether  the  underlying  airspace  is  controlled 
  or  uncontrolled - may  not  exceed  the  200-knot  speed  limitation


So it is indeed any and all airspace underlying class B.

And note the exception to the rule:


  We  note  § 
  91.117  (d)  does  provide  that,  if  the  minimum  safe  airspeed  for  any  particular  operation  is  greater  than  the  maximum 
  speed  prescribed  in  § 
  91.117,  then  an  aircraft  may  be  operated  at  that  minimum  speed.