It rather depends on the type of illustration involved. If they are both photographs then I think you assesment is correct. However if they are engravings then the more recent illustration from 1951 will probably still be in copyright.
The reason there is a difference, is that in the 1911 Copyright Act, photographs were in a special category of their own with a fixed fifty year term of protection. Other forms of artistic works such as paintings, drawings and engravings had a term of protection which was based on the lifetime of the artist plus fifty years. Therefore assuming the artist who produced the 1951 illustration was still alive when publication occurred, the copyright term would have lasted until his death plus 50 years, meaning that the work would still have been in copyight in 1995 when an extra 20 years was added to the post mortem part for any work then still covered by copyright. As you can reasonably certain an engraving or drawing from 1951 is still in copyright today, you would need to obtain a licence to use it from the Mary Evans Agency