There have many changes (over time) to POWYS and the sub districts associated with it.
Perhaps the following will assist you.
Welshpool in Montgomeryshirehttp://www.fhsc.org.uk/genuki/REG/mgy.htmMontgomery
Created 1st July 1837. Abolished 1st July 1870 (renamed Forden). Mainly in Montgomeryshire, but included parts of Shropshire.
Sub-districts : Chirbury; Montgomery; Pool.
GRO volumes : XXVII (1837-51); 11b (1852-1870).
Aston, Berriew, Castle Caereinion, Castlewright, Churchstoke, Cletterwood, Cyfronydd, Forden, Guilsfield (1837-56), Hope, Leighton, Llandysil, Llanmerewig, Middletown, Montgomery, Rhosgoch, Trelystan, Trewern, Uppington, Welshpool (Pool).
Registers now divided between Welshpool & Llanfyllin and Clun districts.
http://www.fhsc.org.uk/genuki/REG/rad.htmMid Powys includes the followingBuilth
Created 1st July 1837. Mainly in Breconshire, but included parts of Radnorshire.
Sub-districts : Abergwessin; Builth; Colwyn; Treflis.
GRO volumes : XXVI (1837-51); 11b (1852-1930).
Aberedw, Bettwsdisserth, Cregina, Diserth, Llanbadarnygarreg, Llandrindod, Llanelwedd, Llansaintfraed in Elvel, Llanvareth, Rhulen.
Registers now in Mid Powys district.
Mid Powys Powys County Hall, Llandrindod Wells, Powys, LD1 5LG.
Tel: 01597 826000 ext 6373.
http://www.northwalesbmd.org.uk/marriageinfo.htmlThe Superintendent Registrar,
Machynlleth Register Office,
11 Penrallt Street,
Machynlleth,
Powys.
SY20 8AG
UK The Superintendent Registrar No
Powys - Newtown District
Newtown Register Office The Superintendent Registrar,
Newtown Register Office,
The Park,
Newtown,
Powys.
SY16 2NZ
UK The Superintendent Registrar No
Powys - Welshpool and Llanfyllin District
Welshpool Register Office The Superintendent Registrar,
Welshpool Register Office,
Neuadd Maldwyn,
Severn Road,
Welshpool,
Powys.
SY21 7AS
UK The Superintendent Registrar