The message
from "Roger Mills" contains these words:

Stelrad single panel?

23" high.

88" long 5921 Btus/hour.
82" 5498
76" 5075
69" 4652
63" 4229
57" 3808
50" 3384
44" 2961

At what Delta-T value?

Those outputs look a bit optimistic to me - even at a delta-T of 60 degC
(140 degF) my calcs come out a bit lower than that.

Figures were taken from the Ravensbourne Guide to D.I.Y. Central
Heating. It is not dated but I think it is perhaps circa 1975 judging by
the prices and my contacts with the firm. It doesn't say how the figures
are calculated but a later offering from Ravensbourne (probably circa
1988) does state "Manufacturers give emission figures for each radiator
at a temperature difference between water and room of 60C or 108F".

The 88" radiator above gives 2.93 btus/sq. inch. The later catalogue
give the closest Barlow radiator - 24*84 as 6105 which at 3.02 btus/sq.
inch is higher but might not be significantly so as Stelrads dimensions
include the seam top and bottom and the Barlow is a roll top. Ignore the
seams and the average output is likewise 3.02.

--
Roger Chapman