Well I know the one I have (literally) sitting about is a 305.50 so a quick google on "305.50" and "alfa engine" in google gives :
http://www.alfisti.co.uk/BoxerA-Z.htmI've used this site before, probably pretty reliable and the above page has the following list of engine codes (amongst other info) that would seem to point to it originally having been injected :
Engine Number Identification-. 301.02 - Single carb Sud 1.2 63hp
301.04 - Sud 1.2 ti 68hp
301.02/S - Sud 1.2 60hp
301.84 - Sud 1.3 74hp
301.60 - Sud Super 1.35 70hp
301.64 - Sud 1.35 75hp
301.68 - Sud 1.35/Sprint 63hp
301.24 - Sud 1.5 ti/Sprint 83hp
301.28 - Sud 1.5 ti/Sprint 95hp
301.46 - Sud 1.5 ti/Sprint/33 105hp
310.00/305.00 - Arna 1.2 60hp
310.10/301.68 - Arna 1350 Gti 85hp
310.16/305.20 - Arna 1500 Gti 95hp
305.00 - 33 1.2 68hp
305.02 - 33 1.3 75hp
305.04 – 33 1.5 ti 84hp
305.20 – 33 1.5 ti 90hp
305.50 – 33/Sprint 1.7 114hp
305.58A - 33 1.7 ie Kat 105hp
307.43 – 33 1.2 78hp
307.32 – 33 1.3 86hp
307.34 – 33 1.5 105hp
307.50 – 33 1.5ie 96hp
307.51 – 33 1.5ie Kat 95hp
307.46 – 33 1.7 16v 137hp
307.36 – 33 1.7 ie 110hp
307.37 – 33 1.7 ie Kat 110hp
307.47 – 33 1.7 16v Kat 132hp
307.55 - 33 1.3/1.4 IE specific to Greece and runs the Bosch L-Jetronic 3.2
335.01 – 145/146 1.3i Kat 90hp
332.01 – 145/146 1.6i Kat 103hp
334.01 – 145/146 1.7i 16v Kat 129hp
Cheers
Paul.