Please report back when you have received the new belt.
There are sellers that claim that their belts will work but they only considered
the belts size when producing them and not the elasticity.
The elasticity is particularly essential in these decks since the belt should of course
have a good grip around the platter and the idler wheel but at the same time it should also
allow the idler wheel to swing away from the motor pulley with only little force when changing speeds
and accept that the idler wheel sits farther away from the platter at higher speeds.
Without the correct elasticity, one function won't work.
So make sure you can change speed (33/45 etc.) with the new belt.
I can supply belts for most Beograms but I haven't yet found a producer capable of
making suitable belts for this Beogram series.
Size and friction is not the problem, the elasticity is.
When you have the old and new belts side by side, you can easily tell if the new belt will work.
I've had lots of requests from Beogram owners, who bought a promised-to-be-suitable belt on Ebay etc. that,
when fitted, turned out to be no good but I couldn't help them either.
I think the sellers genuinely believe their belt works but they haven't tested it themselves.
If everything else fails, ask B&O. They reproduced the round (cross section) belts a couple of years ago,
not cheap but I suppose they will work.
Let us know your results.
Martin