If you are reading fob type from the plastic case, then do not do that. You need to open the case and look what is written on the PCB.
Fobs 3TXB, 3TXC, 3TXD and 17TN are of the same type and can be mixed without any problems. Only difference is 3TXA.
Any link to cut off switch as I am intrugued where it is mounted when you say you have smaller negative post and bigger positive post?
If it is battery terminals both battery terminals (positive and negative) are the same size in my case.
Firstly this thread is posted in wrong section, so I assume Admin/Mod team will move it to correct section.
To answer your question. No it is not possible to do such thing. This is security feature and if alarm box is immobilized you cannot force the Engine ECU to learn code from such alarm box. You also cannot force the alarm box to unimmobilize itself other but with use of appropriate matched fob.
Otherwise it would be pretty easy for thiefes to put another engine ECU pair it with alarm box and drove car away.