BT-56 is Centuri ST-13. See the Semroc section of eRockets......
Actually not quite...
ST-13 is listed as 1.340 OD, 1.300 ID.
BT56 is listed as 1.367 OD and 1.346 ID.
Effectively the same but not interchangeable if you wanted to make a repair. eRockets does list several balsa nose cones for ST-13, none for BT-56
Shouldn't the BT-56 be 1.346 OD and 1.304 ID, just to clarify?
Some time ago I was told BT-56 was the same as ST-13....so I've never looked for BT-56 dimensions. I sit corrected.
now, yes, it has finally been discontinued.