The circa 1998 prototypes used a compensating valve block with a bore larger than even the one used for the 993 and 994 BBb compensating tubas. My galleries hold this documentation:
Thumbnails of three samples of a circa 1998 prototype for a Besson 985 F tuba with 4 front action compensating pistons, a model that never was set into production. The dimensions can be read from the caption of the first picture:
http://photos.groups.yahoo.com/group/yo ... ba+4P+comp
http://photos.groups.yahoo.com/group/yo ... F+tuba+4FP
If I should comment on the resurrection of the 75 years old Besson F tuba, then I would say, that there is a fairly easy road to go:
The old valve block had the same bore dimensions as the one of the present 981/982 Eb tubas. The modern valve blocks can have their valve tubing cut down to F lengths without too much trouble.
On diverse online auctions one often sees period Eb tubas offered. Often with accessible price tags, as the valves are shot.
Get such a carcass with a suitable wrap, cut some cylindric tubing, and mount a modern and modified valve block.
The last decision to be taken: Which leadpipe?
If you want to play with the original small bore shank, then get a spare leadpipe for the old Imperial/New Standard Eb.
If you want a large modern leadpipe allowing modern standard mouthpieces, then take a leadpipe from a 981 or 982 Eb.
As much as I prefer the fast expanding 981 leadpipe for my modern purposes, I think, that it would defy the purpose of this project. So the slower expanding 982 leadpipe probably would be optimal for this project.
Klaus
PS: As obvious from the addresses given above and from the signature lines below here my galleries are organised in the form of a series of Yahoo groups. By definition (from Yahoo) stored files and photos, close to 50MB in each group, are only available to group members.
But then these groups are welcoming virtually everybody.