It's impossible to say if any random luthier made 5 string conversion neck for a "flat head" would fit correctly on a ball bearing rim. It all depends on where they luthier places his lag bolts. Because the flat head rim and ball bearing rim are not exactly the same dimensions.

I had Jimmy Cox put one of his flat head rims on my 1926 ball bearing rim. Everything worked out "close" to OK. Except when all finished my assembled put was 0.10" (one tenth of an inch) taller than it used to be. The fingerboard consequently was 0.10" too low compared to the plane of the head. I altered the holes in the rim to raise the fingerboard up to where it needed to be.

Then there are "full height" flat head tone rings (on relatively short rims) and "no cut" arch-to-flat-head tone rings which fit on tall arch top rims. Anyone who made conversion necks for such banjos might put their lag bolts in a place that didn't work out just right.

Usually this neck fit is sort of made to measure and inspected for proper fit before finalizing lag bolts, I imagine.

And yes, ball bearings do have skinnier lag bolts and nuts, than post 1927 Mastertones. It would commonly call for switching to the later type coordinator rods.

Luthiers save up years of experience and know how and "surprises" to properly fit necks to pots. Not like IKEA parts.

It's more important if a neck is cut for a tube-and-plate flange or a one piece flange, than it is style of tone ring.

Genuine Gibsons ball bearings had tube-and-plate (2 piece) flanges.

Flathead rings are more commonly associated with one piece flange rims, but not always.

Run lots of searches about ball bearing mastertones before you make any decision. Prewar use fine thread small hanger bolts. Most conversion to five strings use modern larger hanger bolts with new dual coordinator rod setup. The stock prewar ones from tenor setup could be used, but easy to cross thread. If I were you, and bought a prewar ball bearing mastertone not already converted with 5 string neck, I wouod leave the original tenor neck intact in the original hard case it will come in and leave that stuff in closet. You do not want to try and reuse century old Grover two tab tuners. You will not be able to stay in tune, don’t reuse them.
Run lots of searches to educate yourself what can be done as the stock tone tube assy can be flipped for flathead look. I just received a super old Washburn banjo. Despite being involved in banjos since 77, I’ve spent a few hours running searches so far. If you’re into a ball bearing, many want a flathead conversion tone ring. That’s another set of issues, despite some claiming “drop-in”, it may not be. Lathing “may” be required, then you can’t go back. Research, research, research, or…..risk decisions that are permanent in some cases.
I know in case of my 26 ball bearing, I inquired about a flathead conversion ring, and found no one interested in selling one without me sending my rim for “minor work to ensure proper fitment”. What that means is many ball bearings had a tiny wooden ledge invisible until total disassembly. That little ledge positions your stock tone tubes assembly on the rim. Once lathed away, if you ever wanted to go back, your outer skirt for tone tubes assembly will not be fit as it had stock. You would have issues then lining stuff up.
To avoid this on my 26, I removed metal from the drop in conversion ring I bought so the ring seated without removing stock little hidden wooden ledge. So I can return mine to stock and have perfect functionality and no alteration to bottom end.
Most people who talk ball bearings had a have not disassembled one, and this tiny wooden ledge is not there on all of them. Ball bearing is the cheapest prewar mastertone, and has like a tone tube, tone rod, and tone ring skirt fashioned into the “tone ring assembly”, and it looks nothing like a cast flathead ring commonly seen. Under the brazed tone tube assembly when removed is series from bottom of hole upward orcwasher, hell to compress motorcycle valve spring,ball bearing. I don’t recall how many but a lot. If you ever wanted to completely disassemble ball bearing, you need to do so on a white sheet on the floor. The pieces are bad to want to run away and hide.
Somewhere in all my photos is complete pix of total 26 ball bearing mastertone disassembly. Be careful of your hooks, the flat hooks bad to want to snap the head off.
Reuse your stock tenor grover Presto patent pending tailpiece either by using two string loops on one tab, or drill 3rd string hole in Presto, or have Presto punched for 3rd string hole at machine shop, or run third string UNDER tailpiece. You won’t find a better sounding tailpiece than stock prewar Presto.
On mine, I do wish I’d went ahead with aftermarket notched tension hoop and regular round hooks, but it’s done now and I doubt it’ll ever come apart again by my hands. Im not a split tension hoop or flat hook fan. But, Thats cosmetics. The flat hooks are pain in the butt wanting to snap their heads off so be gentle.

the guy i got my 27 tb3 put a goldstar neck on it i have a 94 earl eric told me he made it really nice job

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Originally posted by Brett

You won’t find a better sounding tailpiece than stock prewar Presto.

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This should be mentioned way more often!

Nothing beats an old Presto.