Advice on vintage Onoto nib sizes and interchangeability?

[velox brunneis vulpes]

Dear fellow FPNers,

I have recently acquired 2 vintage Onoto plunger fillers, a model 3000 with a No.3/ST nib, and a model 6000 with a No.3 nib.

Could someone please let me know if the nibs are interchangeable?
And what does the "ST" on the No 3/ST stand for?

I look forward to your feedback

Thanks in advance!

 

Best,

J

[felix_classicpens.uk]

St stood for stenographer. They were firmer (less flex)