He finds Milliways after Itama dies, after Hashirama's no longer talking to him because Tobirama broke up his friendship with that Uchiha (saving him, not that Tobirama's brother is grateful).
It's clearly not the same dimension as his home.
He's far, far from the clan compound, on a mission alone in enemy territory, and he has no one to call to.
He sleeps in the backyard, forages for his food, visits the infirmary, and talks to Bar.
He's... More tempted than he should be, to leave and never, ever return. To stay years and years, even, until he's older and people won't see some kid they can ignore as they send him off to fight...
But Tobirama has his duty.
So he talks to Bar, and she says she can identify his alts, and she can give them a book.
He thinks long and hard about what to put in it, eventually settling on:
My name is Senju Tobirama. If Bar has given you this book, she has identified you as an alternate universe version of me. I cannot stay long, here, but if I ever find this place again - especially as an adult - I intend to stay longer, and collaborate with my alts if I can. The book can be updated, so if your status changes you can alter details, though if it's been years I would recommend a new page, for record purposes.
To this end, please leave the following information: Name, Alias, Species, Occupation, Age, Stage of Development, Notable Life Events, Notable Traits, World of Origin, Current World, World Synopsis (include powers, technology, political overview, and note if your world is a variant on another), Notable Family, Notable Allies, Notable Enemies, Messages to Alts, Requests from Alts, Other Not Specified, Image (Bar will provide you means)
Name: Senju Tobirama (Family name then given name)
Occupation: Shinobi (see 'World Synopsis' below)
Age: Ten Earth standard years
Stage of Development: Pre-pubescent
Notable Life Events: I've been a shinobi for four years. I lost two younger brothers within the last year. I had a falling out with my brother because he befriended an enemy one month ago. My father is planning to send me to another nation as a hostage for four years, in return for a marriage and alliance contract for my older brother (I plan to learn their magic).
Notable Traits: I am albino. I have poor vision. I have water natured chakra.
World of Origin: Rinne (my name for it)
World Synopsis: My world's main magic system is chakra. Chakra users are monks, shinobi, or samurai. The properties of chakra are as such... My world is politically in a Warring States period, after the fall of an Empire. Recommended reading about my world is...
Current World: Rinne
Notable Family: Senju Hashirama (older brother; he's got an annoying personality...). Senju Itama (younger brother; deceased). Senju Kawarama (younger brother; deceased)...
Notable Allies: I have no allies outside my clan
Notable Enemies: Uchiha Madara, who is...
Messages to Alts: I believe my magic system is more flexible than average, and, given enough time, I should be able to solve many problems.
Requests from Alts: I need a way to bring back the dead, to change my apparent age, immortality, and ways to end wars
Other Not Specified: I haven't thought of it
Image Description: ...