pair of Adam chairs ...Research?

Hello we got these circa 1770 period Adam style armchairs over the weekend and would like to find out more about them. So far we have a tale spun by a NY dealer back in '95 that tie this pair to a suite they were marketing consisting of 4 chairs and a settee. He tied the suite to Lansdowne House London by virtue of a stamped IS on the pieces. The logic was that research showed the IS was an upholsterer's mark and that he had supplied goods to the house as it was being outfitted ( the research didn't reference these chairs). I have tried to research online but don't seem to be finding much about the house or the dispersal of it's contents. Any suggestions on the research and or value would be GREATLY appreciated. Pics are of the lesser of the two. Thanks!
Roger

 

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Underside, bottom, feet and structural photos needed please, plus this stamped IS. It's been regilded: my gut feel is that they're later copies.

 

pics of underside as much as I could (the boss said I couldn't take webbing down).
There is evidence of past upholstering.

 

Evidence of pegged construction which I believe speaks to the age?

 

I dont see any evidence of woodworm holes,wich Id expect for a piece of this alleged age? They are lovely chairs regardless.

 

Hi, the block shown is an Edwardian winged block. Hope this helps.

 

I agree, a later repro from the late 19th early 20th century. Numerous differences with 18th century original. Pegged holes show earlier construction technique, but these are perfectly round and not raised so not period. Here is a link to a similar museum period chair to compare with (https://i.pinimg.com/originals/00/28/ce/0028ce0635bd1b335c38477d83e09404.jpg):

 

The type of block is very common on Edwardian pieces but here is an 18th cent example

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• About
  A Fine Pair of Louis XV Giltwood Fauteuils Circa 1761-62 By Louis Delanois Maitre in 1761 Stamped Delanois The shaped back with a carved floral crest leading to two shaped scrolling arms, over four cabriole legs with carved floral knees all ending with tupie feet. Height 37 in. Depth 22 in. Width 26 in Similar model and example can be found in the Louvre, Paris. Reference: Le Mobilier du Musee du Louvre, by Bill Pallot, pg.94-95 Le Moblier Francais du XVIII siecle, pg.230-236. Provenance: Private Collection New York Cha158
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  A Fine Pair of Louis XV Giltwood Fauteuils By Louis Delanois, 18th Century
  
  
  
  
  
  
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In my photos the pegs appear more uniform than they actually are on this chair. The pics were done at night and I didn't even see any through the gesso on the other chair. Today in daylight what appeared to me just an irregularity the other night proved to be the only peg telegraphing through and it is a diagonal oval about 1/4"x3/8". I noticed something else today that intrigued me. The arms are mortised into the back rails which I don't see on my other Chippendale furniture (purchased as known repro because my grandfather carved them). Is this a feature on English repros? I have absolutely zero experience with English furniture and this was a "wife impulse".