It is a Honeywell Galaxy 2 series (I believe a 2-20). I have now taken the
sensor off thankfully without setting the alarm off 😀. There are
2 resistors in it. 1 for alarm and 1 for tamper. The markings are brown, b
lack, black, brown,brown. Looking online, depending on which whether you re
ad this forward or backwards you seem to get 1k or 110. Any ideas?