Hopefully someone with a working unit will send you the bulb number. I have seen some medical bulbs listed that might work but there were no specifications other than voltage and price.

www.donsbulbs.com has a lot of information. A sub miniature bulb with a SX4 base and T1 globe in the 3V to 5V range may work. I only saw 9mm length bulbs.

Most led's I've seen are 5mm in diameter, too large. Grain of wheat bulbs are in the 12V or higher range. A bi post or wire lead bulb may be adaptable.

Hello from France again,

I'm still searching for the suitable lamp for my Kalart focuspot for a Rolleiflex tlr 3,5.
Thank you for your help.

Yannick Michel