Back in the 50s,60s,70s, and even into the 80s McIntosh engineering found parts quality to be hit and miss and therefore would list parts they felt dealers should purchase from Mac parts. These parts would have gone through the typical Q/C process that Mac would apply for new production.
Non critical generic parts were expected to be purchased locally and most often the dealer already them in stock.
If you are asking about the ones I think you are they are just generic 1n400x diodes. The generic number should be printed on the part, my guess it is a 4004 but you should confirm.