When I fitted an uprated servo/master cylinder from S&S some 5-6 years ago (think it was a Sherpa unit, bought after problems with a recon cylinder) it came without a reservoir, I assumed because the TR7 one is wedge shaped. The TR7 one slotted in OK. Might be worth trying S&S or Robsport for a secondhand one. James Paddock show a new Lucas TRW mastercylinder complete with reservoir but not the reservoir alone. Might also be worth calling TRW?
Saab 9-5 2.3t Vector Auto Estate Stage 1
Saab 9-3 2.0 SE Turbo Convertible
'81 TR7 DHC