Short  version:  Concerning the black Temp sensor in the Thermostat housing which supplies info to the coolant temp gauge- is there any difference between 251 919 501 A  and  251 919 501 MY other than "A" having a square plug and "MY" having a round one?  

Long version:  In August my coolant gauge suddenly began to read considerably lower than it had previously.  This was a concern but the vehicle performed in every way as if nothing had really changed.  The low speed fan would kick on just as it always had. No more no less. Since it was reading low rather than high I continued using it lightly (short local trips)  watching carefully-everything seemed normal.  At about the same time I started losing coolant through rust blisters on the pipe from the water pump to the therm housing. I ordered a new pipe and new temp senders, blue and black, hoping the fix would be that simple.  Pipe back-ordered, didn't come till now. So, I just replaced the pipe and the sensors, only hitch being the new black sensor turned out to be round plugged 251 919 501 MY and my harness plug was square. Darn. The plug was corroded and while cleaning it the contacts fell apart. Darn again.  But my 87 parts van had a good plug of the round variety so I cut it out and spliced it into the 91 and used the round "MY" sensor.  All back together, no leaks, but now my temp gauge goes higher than it ever did. Seems like it will just keep rising to the upper limit if I let it which I won't.  The fan (which works well hi and low) does not kick in so I am wondering if this is just a sensor problem or something more serious.  Are these sensors that all have the same number but followed by different letters compatible except for the plug type?