Unfortunately, this isn't from Happy Endless. It certainly looks like Tana's work though. Did he do any other Mahou Shoujo Ai doujins? I think it's from the same place as post #63561.
There's 46 pages there, doujinshi.mugimugi.org says the whole work is 48, so this might be one of the two missing ones, or mugimugi might be off by 2. I'm inclined to think it's not one of those for one specific reason: the text in this has a fade border, the text in Happy Endless is in text bubbles.
Found the actual source book for this thing. Mahou Shoujou Ai Colors Anthology 2 ([アンソロジ] 魔法少女アイ colors アンソロジ2). This picture was done by TANA (Garyuh Chitai circle).
It's true. My throat is plugged up. The sensation of cum flooding my throat has completely changed. I can't speak. Even though... it's so very hot... and pulsing. My throat and the inside of my mouth... are pulsing with my heartbeat...The odd-looking penis is rubbing its sticky fluids onto my tongue. And my lips, behind my gums, my upper and lower jaws, and deep in my throat... The penis is thoroughly staining its smell and taste... into me...I have no way to resist, so I swallow it. Traveling down my throat, it accumulates in my stomach. The foul smell and taste... permeates my body... My insides have been defiled...Whoever it is... Even though I expected to hear it, I don't understand why.
(Translation Unsure)Hahaha, that's right. Everywhere this semen has touched has become erogenous zones. Now your mouth is as sensitive as your pussy. As my cum fills up the inside of your stomach, your guts become like a pussy.
(Slightly Unsure)Ngohh... Nbuh, Nguh, Nnohhh!Stop... iiit! Ah! Nnnaaaah! S-So hot... His cock is... g-going inside my stomaaach~! Even though... it hurts, and... even though it's dirty... My mouth... my throat... my stomach... are cumming... The cock's rubbing inside me... My mouth-pussy is cummiiing!However, you don't have to worry in this case, Ms. Magical Soldier. Fufu... Your voice... can't come out, can it?
(Slightly Unsure on last part)Oguh! ...Kuhoh, S-Stop thi... Fugoh! Oehhh!Fuh! Guohhhh~~~! Ngoh, Nguhoehh! Nguh! Nguh! Nguuuu!S-Smelly... Yuragi's penis is inside my mouth... no, inside my head, past my breasts. I'm being filled with the smell of cum... I c-can't breathe...!
(Slightly Unsure)Ogoh... Nguh, Nguuuuu! Nn, Nn!All the cum is coming out at once. It matches the rhythm of my successive attempts to spit out the hot juices. There's suddenly a pool of semen in my mouth.I'm not acquainted with that manner of speaking. It's this mood's fault. Surely... No, absolutely.Uu...? Nguuubuu! Gofuu, Nguu!Fufufu... Why are you so surprised? Is my voice that odd? ...Kukukuku...That sudden voice... Yuragi, who hasn't said a single word up until now, spoke from its gluttonous mouth. However, I am not surprised by such a thing. I'd recognize that voice anywhere.Does it feel good?
