Is it possible to know the date of this publication? At least if it was published on the XIXth or the XXth century?
The preface says: „Though fully 80 years have passed . . .” (that is: since the letters were written for the first time). This fixes the time of publication in the 1950th.
Thank you Dr. Martin, even though Richard Holden lived in Lisbon until his death I couldn't find even one of his writings in the Brethren Assemblies in Lisbon. So whoever published this had access to older writings of his.
Richard Holden's Six Letters to a mother on church questions was published by W.H. Broom in 1873 and Broom published a 4th edition in 1885, a year before Holden's death in Lisbon in 1886. There is a copy of both these editions in the CBA in Manchester. According to Napoleon Noel's History of the Brethren (Denver,1936), Holden's 'letters to his mother, giving reasons for leaving the Church of England, had in its time a wide circulation and is still  obtainable' from the Erie Bible Truth Depot, French Street, Erie PA. (Noel, i.146) Timothy Stunt