I know it amounts to splitting hairs but either my memory is on the blink or you are mistaken about the age of your alarms.
I thought the Super XL’s were the last fling of Optonics in their original guise and they came out in early-mid 90s?
Bought my Super Compacts which were the top of the range of the un-converted Optonics in 87 and between them and the Super XL’s they produced the Magnatronic.
I stand to be corrected but I am sure XL’s were a fair bit later than 87?
Looking on Ebay my guess would be £10-£15 each.