The Answer

All transmitters must have fcc approval in order to qualify to be used for a licensed station. But there were several different standards that were used and different sources name different standards. It would be a big drag to buy a transmitter that met the wrong standard- however, that is no longer a problem as a result of these rules. The below should clarify this.

In 1998, the FCC changed the rules, simplifying the five classes of equipment compliance down to three classes. Some language in older copies of old rules that are randomly cut and pasted here and there may still be around, but the standard for FM broadcast transmitters is clearly type verification. It is no longer type certification or type acceptance. If something was previously approved by any other process, it should meet the new standards, since they are actually much less stringent and require less paperwork with the FCC. The only thing that is definitely no good for low power fm operators is transmitter equipment that is certified under the part 15 rules- that means it operates with flea power. Some scamologists with hearts of crud have been selling "part 15, FCC approved equipment" as if it was a broadcast transmitter. That equipment can only broadcast for a few hundred feet- it is not what you want.

And here is the rule, once and for all, as amended in 1998. Section 73.1660 Acceptability of broadcast transmitters.
(a) An AM, FM or TV transmitter shall be verified for compliance with the requirements of this part following the procedures described in Part 2 of the FCC Rules.

(b) A permittee or licensee planning to modify a transmitter which has been approved by the FCC or verified for compliance must follow the requirements contained in 73.1690.

(c) * * *

(d) AM stereophonic exciter-generators for interfacing with approved or verified AM transmitters may be certified upon request from any manufacturer in accordance with the procedures described in part 2 of the FCC Rules. Broadcast licensees may modify their certified AM stereophonic exciter-generators in accordance with 73.1690. (e) Additional rules covering certification and verification, modification of authorized transmitters, and withdrawal of a grant of authorization are contained in part 2 of the FCC Rules.

If you want to read the full text of the decision it is available at: http://www.fcc.gov/Bureaus/Engineering_Technology/Orders/1998/fcc98058.txt