ancient custom time signature

[karl]

I have a piece with the following time sigs:
 alla breve
 timesig.mensural34 followed by a 3.
 alla breve

How do I typeset the timesig.mensural34 followed by a 3 where
it should be ?

///

I try to typeset [1], on page 6 (101 in the original), there is this
mensuration (time sig.):
 "tempus perfectum dimunitio et proportio tripla"
i.e. a timesig.mensural34 followed by a 3.

With the help of [2], I have made:

#(define ((proportioD glyph a b) grob)
   (grob-interpret-markup grob
          (markup #:override '(baseline-skip . 2.5) #:number
                  (#:line ((markup (#:fontsize 4 #:musicglyph glyph))
                           (#:fontsize -1 #:column (a b)))))))

#(define ((proportioS glyph a) grob)
  (grob-interpret-markup grob
   (markup #:override '(baseline-skip . 2.5) #:number
    (#:line ((markup (#:fontsize 1 #:musicglyph glyph)) #:vcenter (#:fontsize 
-1 a)) ))))

///

With
  \override Score.TimeSignature.stencil = #(proportioS  "timesig.mensural34" 
"3")
I basically get what I want, but there are two problems:

1, the timesig is 25% bigger with this function

2, the timesig doesn't appear where I want it, it happens at the "next" 
   time sig change (where there should be an alla breve instead),
   not at "this" (\once doesn't help).

My code is at [3], the current result is at [4].

Any help would be appreciated.

Regards,
/Karl Hammar

[1] http://turkos.aspodata.se/choir/ensemble/palestrina.stabat.mater.pdf
[2] http://lsr.di.unimi.it/LSR/Item?id=725
[3] http://turkos.aspodata.se/git/musik/Pierluigi_da_Palestrina/motetten_a8/
[4] http://turkos.aspodata.se/choir/ensemble/Stabat_Mater_a8.pdf

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