I just spoke to one friend and we're in agreement... that doesn't look like any Toyota bellhousing either of us has ever seen. That's not saying much coming from me, but coming from Jeff it's almost a guarantee.
The closest thing I've been able to find is a bellhousing for a Dodge Dakota 2.5L which is used by some to install R154s and similar transmissions into certain other cars... but, the starter is on the opposite side. It has the same venting at the upper rear and the same extra large weep at the bottom.
http://www.3800pro.com/forum/rwd-380...html#post40010
I'm of the impression that that R154 had already been swapped into something else before it got to you... as such, the only way you're going to place an age or vehicle of origin to it is to take it apart and thoroughly inspect the internals.
You might try contacting a company called Marlin Crawler... they do a LOT of work on R154s and other R series transmissions and may be able to help you further with this.
http://www.marlincrawler.com/
If you find anything else out please let me know, I'm very curious.