I have a theory that RAB is part Regulus and part of somebody else

Cuz regulus couldnt do it alone, he would have to have some help possibly kreacher cuz Voldemort would probably disreguard a house elf as a threat like he disreguarded underage wizards with the locket horcrux

One problem w/ my theory tho is that the letter said "I" not "we"or "us" so the letter refers to only one person not 2 or 3

so like the book knda contradicts itself or I am wrong (it wouldnt be the first time) lol