Can I still buy a TPL121500?

Not as new stock from CSB. The number has come out of the current telecom front terminal range and the TPL121600FR occupies that position now. If you have a partly populated string and you need the old part specifically to finish it, say so on the quote request and a specialist will tell you honestly whether any remains findable, and will name the substitute if the answer is no.

What is the difference between TPL121500 and TPL121500A?

The A is CSB's revision suffix on this line, the same convention that produced the TPL121250A. Our own catalog record for this listing carries the manufacturer part number as TPL121500A, so the label on most units in the field will read that way. An FR suffix on top of it denotes the flame retardant case. All three forms describe the same superseded position in the range.

Will the TPL121600 sit in my existing tray?

Usually, and the reason for the hedge is documented rather than defensive. CSB has revised this case more than once and the width and height printed on older sheets sit a couple of millimeters away from the current figures, which is nothing on most shelves and decisive on a tight one. The current block is 21.54 by 4.94 inches and 12.72 inches tall. Measure the opening, send the number, and it gets confirmed before the order goes in.

Do I have to replace the whole string at once?

Yes, and we would rather lose the smaller order than tell you otherwise. A string charges and discharges as one unit, so mixing an older block with new ones leaves the whole bank governed by the weakest section and pulls the new blocks down toward the old ones. Replacing four of eight saves money this year and usually costs a full string inside two. Budget for the bank rather than the block.