There are welfare fund and Welfare Fund Boards for different activities, say, for employees in construction sector, there is Construction workers welfare fund board, for workers in loading and unloading there is Headload Workers Welfare Fund Board etc. In order to get your establishment and workers registered under the welfare fund scheme, you will have to approach the relevant Board and do the formalities including payment of contributions. What the Principal employer would like to see is whether the workers engaged in his plant are registered under the Fund or not. For that you have to show the contribution card.
Again I would say that establishments to which PF is applicable, welfare fund is not payable. This is because the latter is a state Act and an Act passed by a state legislature applies to such establishments to which a Central Act (Employee Provident Fund and Misc. Provisions Act is a Central Act) does not apply.