All aircraft are certified to a certain MTOW when they are delivered. Generally, the difference in the MTOW is only on paper, therefore allowing customers to customize the 'limits' of their aircraft to suit their own needs. Any increase in MTOW by an operator requires a significant payment to be made to the manufacturer as originally, the aircraft was delivered at a lower designated MTOW, and therefore was cheaper to buy.
The reason that there are "A Market" 777's in United's fleet is because these first aircraft were designated to serve the shorter trans-Atlantic and domestic flights, most under 7 1/2-8 hours in duration, therefore negating the purpose of having the additional fuel tanks and beefed up structure which is present on the -ER model. With the additional structure and fuel tanks on the -ER, the MTOW was subsequently increased, which would have been a great additional cost for United, who didn't even need the -ER features for those routes! (also why the non-ER aircraft have de-rated engines--has to do with MTOW).
All 16 of the "A Market" 777's do have PTV's, as they were the aircraft originally delivered with the GEC Marconi systems installed.
A little more about the 777's for you to give the same point on a different issue:
One of the biggest "wants" on these aircraft now by flight attendants and pilots alike is enhanced crew rest features. Currently, there are a handful of new -ER aircraft with the LLCR (Lower Lobe Crew Rest) for the flight attendants, as well as two bunks just aft of the cockpit for the pilots. Other -ER aircraft have an "enhanced" flight attendant rest in Economy, including 39" pitch, legrest, footrest, lumbar support, and a sound deadening curtain, with a United First Suite reserved for pilots. And lastly, the "A Market" 3-Class aircraft have regular Economy Class seats and a United First Suite seat for pilots.
Both Flight Attendant and Pilot unions would like to see all 777 aircraft equipped with LLCR and main deck bunk facilities, however there are very few routes which contractually require these types of rest areas. Just as in the case of ordering an -ER 777 aircraft to fly a 6-hour long flight, refitting each 777 with these facilities is a waste of money, as the upgrades are pointless (E. Coast-Europe crew rest can be as little as :30).
Hope that clarifies a little more for you!
F L Y 7 7 7 U A L