Practice and race number at Belgian GP

Hello to all.
The high quality of reply and discussions brings me to come with my nighmares.
In the Belgian GP, in the fifties usually, to avoid the printing of "pirate progams" for the
sunday race, the organizers gave other car number for practice.
Has someone official items on this subject?
My particular querry is for the 1958 GP. And the Maserati.
Race N°26 MT de Filippis had n°30 in practice
n°28 Trintignant ?
n°30 M Gregory had n°34 in practice, other question who shared his car during
practice? driver with white helmet...
n°32 W. Seidel had n° ? in practce
n°34 K Kavanegh had n°44 in practice
n°36 J Bonnier had n°38 in practice (?)
n°38 F Godia ? car all red during praactice, yellow on the nose only
painted for race
L practice car presumably piccolo raced by Fangio in ACF GP. Who drove it in practice?
Onne driver wore also a white helmet.
Of course I don't go into chassis numbers, too complicated for me.
Thanks for helping.

I thought Barrie Hobkirk would have answered this, but he's obviously been forced to do something less important today (like working)
So rather than keep marat in suspense, here's my offering
And, sorry, it really needs to be done by chassis number -
2511 Practice: 34/Gregory and 30/Trintignant. Race: 30 Gregory. Scuderia Centro-Sud entry
2519 Practice: not known. Race: 28 Trintignant. Scuderia Centro-Sud entry
2522 Practice: 36/Seidel. Race: 32/Seidel. Scuderia Centro-Sud entry
2523 Practice: 30/de Filippis. Race: 26/de Filippis
2527 Practice: 34 (or possibly, as marat says, 44) Kavanagh. Race: did not start (listed as 34)
2528 Practice: 42 Godia. Race: 38/Godia
2529 Practice: not known. Race: 36 Bonnier
2533 Practice: L/Gregory. Race: not entered (but raced by Fangio at Rheims)

Bonnier may, as marat says, have had 38 on Car 2529 in practice. Or he may have driven 2528 with that number in practice: he was in the process of buying this car from Godia

Now Barrie will probably tell us how wrong I am

Thanks. Can I concluse that Trintignant practiced also Gregory's car ant the piccolo
(the white helmet).

Yes, Trintignant practised the car Gregory would drive in the race (2511)
AFAIK Gregory was the only driver of the Piccolo in practice (in his white helmet). Do you have pictures of more than one driver in it?

Thanks. Definitely Gregory in Piccolo. I ignored that Gregory used also his white helmet that
year.
Has someone written about helmet colours?