Conn 12J vs. 15J

[Tubajug]

For those that have had these side by side, are the inner bows the same? Are the valve tubing layout and the leadpipe shape the only differences?

I ask because I believe these are the horns I would like to keep an eye out for to build a tiny CC tuba of my own. I plan on using two of these to combine into a four-valve horn in CC. I've done some measuring on a 12J my school owns and the inner bows should line up at the right spot to combine with my H.N. White King Eb bell and outer bows to get it pretty close to CC without a lot of extra cutting.

I basically want to know if I'm ok buying either of these horns if I find one (or both) at a good price.

Thanks!