Hey Jean Pierre I tried to repeat this pose with my own hand and I think the artist exaggerated the bend of the fingers. Also the thumb doesn't look to bend to the correct direction. There's two things I like to point out: first when you simplify the palm of the hand into a box ignore completely the thumb. It's too mobile and it's gonna deform the box. Second is when you are studying drawings from old masters or even more old draftsmen you need to understand that this drawings weren't made for others to study they were made in order for them to understand a subject to paint later. So they are gonna have a lot of incongruences. Does that mean that they absolutely suck and you should avoid them? No you study the things they made right and complete the rest with your knowledge today. Like for example this drawing. The anatomy structure is kinda wonky but he did draw the fingers, the palm and the wrist well. I could photograph my hand use as model and use his simplifications and rendering. This way you have the best of both worlds. Keep drawing and best regards

I'm not sure what your intension was with the second drawing, But I'd suggest giving the hand a tighter grip around the cylinder (have the fingers wrap around the whole cylinder) to enhance the feeling of the drawing. Great work, I really like the shading and your finger studies.