Crap. I reread the manual this morning and you're right. LATCH cannot be used after 48, but the tether must be used after 50 lbs. I have a large number of expletives I'd like to insert here. There are now officially NO SEATS that I can use for my 6 year old. NONE. He's already at the top strap level for the Radian, and will outgrow it in probably 6 months or so. The Regent requires tethering, which we do NOT have in the van and cannot afford to have put in. The Apex requires headrests, which the van seat does not have. I can't believe how totally ridiculous this is. All I want to do is safely secure my oldest child in an appropriate carseat. I feel like no matter what I do, there's no solution to this.