The reference is to the Castelbarco tomb at the church of St. Anastasia in Verona.
At Notebook M2 p.118 the Castelbarco tomb is full of ‘extraordinary energy’. There are measurements at Notebook M2 p.118L, continued at Notebook M2 p.119L. Ruskin’s Index at Notebook M2 p.188 cites Verona Book pp.1-7 for the tomb, and there is further measurement and description there.
The following sheets are concerned with this tomb:
Sheet No. 175 ‘The masonry of the side arch is given in ink on No 175 and one of the capitals.’ Notebook M2 p.119. Sheet refers to ‘p.119. M 2’. Tomb of Count Guglielmo Castlebarco (1320) St. Anastasia, Verona. (RL 1636 box) (part).
Sheet No. 175B Tomb of Count Guglielmo Castlebarco. (1320) St. Anastasia, Verona (RL 1636 box) (part).
Sheet No. 176 Tomb of Count Guglielmo Castlebarco (1320), St. Anastasia, Verona. (RF 1641 p.10) (part).
Sheet No. 177 Tomb of Count Guglielmo Castlebarco. (1320) St. Anastasia, Verona ‘No 177 shows the shafts seen from inside I never saw a vaulting rib so carried before.’ Notebook M2 p.120 (RF 1659).
Sheet No. 178 Tomb of Count Guglielmo Castlebarco. (1320) St. Anastasia, Verona ‘No 178. in old references St Anast Gate tomb p.2 Verona. book’ (RF 1651).
In Stones of Venice see:
Works, 9.176 Woodblock illustration (Fig 34) of Tomb of Count Guglielmo Castlebarco (1320) St Anastasia, Verona.
Works, 9.175-6 ‘the masonry of the side arch of, as far as I know or am able to judge, the most perfect Gothic sepulchral monument in the world’
Works, 9.176 (facing) Plate D The Castlebarco Tomb Verona.
Works, 9.340 (facing) Plate XI Plans of Bases, No. 7.
There is also an illustration facing Works, 19.451 [n/a].
See Bradley (1955) p.195 Letter 155, 24th February  ‘my most beloved- at St Anastasia’.
For a photograph from Ruskin’s Rudimentary Series of the Castelbarco Tomb see here.
A more recent photograph shows it in relation to the church:
For details of photograph see here
[Version 0.05: May 2008]