tci 904 or built 727

They have steel drums, and aluminum ones. The steel is because the wear surfaces of the drum does get worn with every engagement and disengagement, and all the material goes thru the oil system. Aluminum is for minimum ET. It also means replacing it every year or two.
As for where it is and the whys... The way the geartrain works, is the sprague is the pivot point at the back of the case for first gear and reverse. There are two sets of gear sets called planetaries that are splined to various parts to get the reverse, first, and second gear ratios. In 3rd they are all coupled together for direct. One of those parts is a big cupped looking thing called the sun shell. That is splined to the forward clutch retainer. If you pull the valve body, you can see the sun shell and the 2nd gear band over the clutch drum directly over the valve body. The 2nd gear band surrounds the drum. To understand how this all comes together gets a little wordy. I'll try and be brief. You need to realize low gear when in drive, and what's called break-a-way first are not entirely the same. When the shifter is in drive and the trans is in first, the Low/Reverse band is NOT applied. This means the only thing holding the parts steady is the sprague. When the shifter is in "Low" or "1", this is known in Mopar literature as Break-a-way first. In that mode, the Low/Revers band IS engaged as a secondary holding device backing up the sprague. Manual valve bodies do not use the Low band in break-a-way first at all. Because the use of that band means 2nd gear cannot apply as fast. Which is why every box reads "Positively NOT for street use". In first gear, if the tires at any time spin faster than the engine (like letting off suddenly in first gear burnout...) or if the driveline (rear end or Ujoints) fails, the sprague alone is placed under forces it's not designed to take. Some of this can be addressed by using a bolt in sprague, but they dont address everything.. Only some of the forces. When the sprague fails as that holding role, that sintered metal forward clutch retainer (or drum) is allowed to spin at 3 times the rpms it was designed for. It explodes with enough energy to completely explode the case, floor pan, doors, roofs, drivers, and spectators.
When you hear "low band apply" manual valve body, that's the difference. The band needs time to release so inevitably they will be a slower first to second upshift. On a pure race car, totally scienced out and repeatable, this might mean as much as a 2-3 hundreths increase in ET.
This is why I said the only way to address the explosion issue is to replace the part that fails with catastrophic energy. That's the drum, and both 904s and 727s use that powdered metal drum. Most racey 904s use that steel or aluminum drum.
Sorry for the wordiness.