Fixed assets Attachment API

Hello Experts,

I could not find a way to work the attachment API and Fixed Assets. I tried several Business Objects (BUS1022, BUS1056, BUS2040...) also different keys (CompanyCode+Asset+Sub, with and whitout Zeros) with no luck so far. I'm trying to read the attachment count as a first test:

- https://myxxxx-api.s4hana.ondemand.com/sap/opu/odata/sap/API_CV_ATTACHMENT_SRV/GetAttachmentCount?BusinessObjectTypeName='BUS1056'&LinkedSAPObjectKey='XXXX20000017-0000'

- https://myxxxx-api.s4hana.ondemand.com/sap/opu/odata/sap/API_CV_ATTACHMENT_SRV/GetAttachmentCount?BusinessObjectTypeName='BUS1056'&LinkedSAPObjectKey='20000017-0'

- https://myxxxx-api.s4hana.ondemand.com/sap/opu/odata/sap/API_CV_ATTACHMENT_SRV/GetAttachmentCount?BusinessObjectTypeName='BUS1056'&LinkedSAPObjectKey='XXXX000020000017-0000'

Any ideas on the right keys for my request ?

Thank you