What is this part for?
 

It came in the swap i picked up and i cant tell if its a s4 or s5 water pump. What's the module on the side(see first pic). Boost activated or something?Attachment 641904Attachment 641905

It's not an s4 or S5 part.
It looks FD.

its for a cosmo. i can't tell 13B or 20B from those pics

Is that good or bad? Usuable my my 87 n/a?

NOT usable on an 87 N/A...besides the pulley is a Ribbed pulley and you need a dual V belt pulley.
So really you just got a nice Waterpump from another engine that you can't use.

(it would be Usable IN an 87 N/A if you had a 13B Cosmo engine and was installing that engine IN your N/A chassis though.)

It was for my 13bt series 5 swap.

Its a 20B dual thermostat house and water pump, vacuum/boost controlled extra thermostat.
Sell it in the 20B parts section or swap it for the part you need, Id take it but I live in Europe.