I don't know about the .375 350gr bulets, but the .416 410 gr bullets did have their jackets strengthened.

From Woodleigh's web site:

Comparison of MK1 (Cat. 37 on the left) and new MKII (Cat. 37A on the right) jacket thickness of 416 Rigby 410gr SN.

416 Rigby 410gr MKII (Cat. 37A) projectile recovered from buffalo. Retained weight 97% when fired at 2300 fps.

http://www.woodleighbullets.com.au/

-Bob F.