Pretty sure it is Michael Tolkien's - but cannot be 100% sure
I did research it some time back and see that he did use the monogram "MTR" on signing some books and did see photo examples at the time.
Also when searching quickly online now I found this extract from an old ebay listing for a job lot of Michael Tolkien signed books
"Where book is signed by MHRT, unless noted otherwise it is in blue ink, with some capitals in red ink, "Michael H. R. Tolkien". Mostly in English, with few books in other languages as noted. Editions of general books not stated, just printing year. Where dated by MHRT, date in []brackets after book. Some additionally monogrammed MTR (though is MRT)"