trigger assy question????????

[carlos velez]

i bought this sa trigger assy from another website. when it came today i can't even press the trigger guard to lock on to the norc receiver. i didn't force it it might result into damaging the unit. i was told it came from a sa m1a and only fired 120 rds. i examined the hammer it says sa. on the side of the trigger housing i can't read there's a serial no. is this a original trigger group? how come the trigger guard is so hard to push down so it will lock on to the receiver. i see like glued rush inside the trigger. i would appreciate any info you guys can provide me. pls. help me.........................carlos