I managed to secure a copy of the hacked material and was browsing the source code when I came across this comment.
;
; PLOTS 'ALL' REGION MXD timeseries from age banded and from hugershoff
; standardised datasets.
; Reads Harry's regional timeseries and outputs the 1600-1992 portion
; with missing values set appropriately. Uses mxd, and just the
; "all band" timeseries
;****** APPLIES A VERY ARTIFICIAL CORRECTION FOR DECLINE*********
;
yrloc=[1400,findgen(19)*5.+1904]
valadj=[0.,0.,0.,0.,0.,-0.1,-0.25,-0.3,0.,-0.1,0.3,0.8,1.2,1.7,2.5,2.6,2.6,$
2.6,2.6,2.6]*0.75 ; fudge factor
Im unsure of the language but seems very strange to be applying corrections to raw data.
In directory document/harris-tree/briffa_sep98_e.pro