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How bizarre. I have never seen the 5th identified as such by Miraphone. On their older horns, the top valve in the stack was always "." so that a five banger was marked

5 = .
1 = ..
2 = ...
3 = ....
4 = blank

Regardless of how many valves were used, the lowest one in the stack was not marked, as it would be superfluous. They were not concerned with the valve's function when assembling them, only with their physical location in relation to one another.

I have no idea at all what the letters mean, and they may have been added by a technician to help him keep them separate from one another in his head during reassembly.

It's pretty weird that 5 was not marked with a single pip if it is in its traditional location.

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This doesn't follow your post, but it's kind of related and a little bit funny.

Back when I was a mischievous college boy, there was a tuba player who none of us cared for and who got to have the brand new 186 C tuba that the school bought. (btw... It wasn't jealousy related: I had one that I bought myself with guitar gig money... I got it from Giardinelli - gray market - with no serial number and spelled "phone", in the mid-70s.).

It was back during the time that all four rotors didn't turn the same direction. We swapped out one and two for three and four, so the instrument played exactly backwards. All of the notes in the up positions of the regular four valves were down and - of course - all of the downs were up. It took a while for someone to figure out what had been done to it.

For a week or so, they had to play one of the leaky old (repair shop ruined) three valve compensating Besson BB-flats.

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If it's a B-flat, the fourth valve is an F valve, yes?

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