The earlier short alloy clock holders are just that and not chromed. You can, if you wish polish the black ones. I believe that short ones are available new but the form is not as good as the original period items.
Smiths clocks will fit in both types but the long ones were introduced to accomodate the deeper Veglia clocks which first appeared in 1973. The Veglias had a trip function and were initially listed for the Interstate but all models were fitted with the long black holders.