I'm still not sure. There are plenty of accusations, but few actual analyses. Such as these ones. They're particularly useful because they were taken in pubs in, shall we say, the less posh parts of town. Exactly the sorts of places you'd expect adulteration to be performed.
"ANALYSIS OF LONDON PORTER.
In an article in the Daily Telegraph, on a "Pint of Beer," by "Our Own Commissioner," there is given an analysis of seven pints of porter taken from the very lowest of the beer family - common vulgar fourpenny, The following is the report:-
April 29, 1871.
RESULTS OF ANALYSES OF EVEN SAMPLES OF PORTER,
Percentage of Real Alcohol by Weight Coculus Indicus, Picric Acid, or Copperas Common salt M. (Bermondsey) 5.25 Neither Yes T. (Shadwell) 4.5 Neither Yes O. (Spitalfields) 5.5 Neither Much H. (New Cut) 4.75 Neither Very much Q (Shoreditch) 4 Neither Yes F. (Whitechapel) 4.5 Neither Very much L. (Kent Street, Boro') 4 Neither A little
Adulterated porter is commonly three parts or less porter and one part water, the resulting weakness in quality being masked by the addition of colouring matter, brown sugar, and bitter drugs, one of which produces lethargic stupor. I am of opinion that these samples have not been so adulterated. (Signed) JOHN BROAD,
Pharmceutical and Practical Chemist.
It may be as well to mention that the above-named gentleman, to make assurance doubly sure, in a matter of such importance, submitted portions of each sample to Professor Attfield, of the Pharmaceutical College, whose return precisely agrees with Mr. Broad's."
Western Mail - Friday 05 May 1871, page 2.
It looks to me as if the samples haven't even been watered. Whitbread Porter was about 5.25% in 1871. But remember that's ABV, while the figures in the table are ABW. In ABV, they range from 5% to 6.5%. If anything, they look on the strong side for Porter of that period.
What does this tell me? That, other than salt, there wasn't likely to have been much added to the Porter you bought in a London pub. I must say that I'm pleasantly surprised.