My trusty Chambers Dictionary defines a fund-holder as one who has money in the public funds. Funds are permanent Government debts paying interest. An annuity is a payment (generally of uniform amount) falling due each year during a given term, such as a period of years or the life of an individual, the capital sum not being returnable. There are various types of annuity, five in all, called Certain, Complete, Contingent, Curtate, and Deferred or reversionary.
Your mention of Corgarff Castle brought back memories of the Corgarff Youth Hostel which I stayed in way back in the 1960's. A website states that Jennie Smith bought the youth hostel in the late 1970's which is now her cottage, and the nearby bothy.