diff --git a/src/wireviz/wv_bom.py b/src/wireviz/wv_bom.py
index b70a93ac..47aebc10 100644
--- a/src/wireviz/wv_bom.py
+++ b/src/wireviz/wv_bom.py
@@ -9,13 +9,18 @@
 from wireviz.wv_gv_html import html_line_breaks
 from wireviz.wv_helper import clean_whitespace
 
+BOM_COLUMNS_ALWAYS = ('id', 'description', 'qty', 'unit', 'designators')
+BOM_COLUMNS_OPTIONAL = ('pn', 'manufacturer', 'mpn')
+BOM_COLUMNS_IN_KEY = ('description', 'unit') + BOM_COLUMNS_OPTIONAL
+
 BOMKey = Tuple[str, ...]
-BOMColumn = str  # = Literal['id', 'description', 'qty', 'unit', 'designators', 'pn', 'manufacturer', 'mpn']
+BOMColumn = str  # = Literal[BOM_COLUMNS_ALWAYS, BOM_COLUMNS_OPTIONAL]
 BOMEntry = Dict[BOMColumn, Union[str, int, float, List[str], None]]
 
 def optional_fields(part: Union[Connector, Cable, AdditionalComponent]) -> BOMEntry:
     """Return part field values for the optional BOM columns as a dict."""
-    return {'pn': part.pn, 'manufacturer': part.manufacturer, 'mpn': part.mpn}
+    part = asdict(part)
+    return {field: part.get(field) for field in BOM_COLUMNS_OPTIONAL}
 
 def get_additional_component_table(harness: "Harness", component: Union[Connector, Cable]) -> List[str]:
     """Return a list of diagram node table row strings with additional components."""
@@ -50,7 +55,7 @@ def get_additional_component_bom(component: Union[Connector, Cable]) -> List[BOM
 def bom_entry_key(entry: BOMEntry) -> BOMKey:
     """Return a tuple of string values from the dict that must be equal to join BOM entries."""
     if 'key' not in entry:
-        entry['key'] = tuple(clean_whitespace(make_str(entry.get(k))) for k in ('description', 'unit', 'manufacturer', 'mpn', 'pn'))
+        entry['key'] = tuple(clean_whitespace(make_str(entry.get(c))) for c in BOM_COLUMNS_IN_KEY)
     return entry['key']
 
 def generate_bom(harness: "Harness") -> List[BOMEntry]:
@@ -130,8 +135,8 @@ def get_bom_index(bom: List[BOMEntry], target: BOMKey) -> int:
 
 def bom_list(bom: List[BOMEntry]) -> List[List[str]]:
     """Return list of BOM rows as lists of column strings with headings in top row."""
-    keys = ['id', 'description', 'qty', 'unit', 'designators'] # these BOM columns will always be included
-    for fieldname in ['pn', 'manufacturer', 'mpn']: # these optional BOM columns will only be included if at least one BOM entry actually uses them
+    keys = list(BOM_COLUMNS_ALWAYS)  # Always include this fixed set of BOM columns.
+    for fieldname in BOM_COLUMNS_OPTIONAL:  # Include only those optional BOM columns that are in use.
         if any(entry.get(fieldname) for entry in bom):
             keys.append(fieldname)
     # list of staic bom header names,  headers not specified here are generated by capitilising the internal name